tillitis-key/hw/application_fpga/fw/README.md

# Firmware implementation notes

## Introduction

This text is specific for the firmware, the piece of software in TKey
ROM. For a more general description on how to implement device apps,
see [the TKey Developer Handbook](https://dev.tillitis.se/).

## Definitions

- Firmware: Software in ROM responsible for loading, measuring,
  starting applications, and providing system calls. The firmware is
  included as part of the FPGA bitstream and not replacable on a usual
  consumer TKey.
- Client: Software running on a computer or a mobile phone the TKey is
  inserted into.
- Device application or app: Software supplied by the client or from
  flash that runs on the TKey.

## CPU modes and firmware

The TKey has two modes of software operation: firmware mode and
application mode. The TKey always starts in firmware mode when it
starts the firmware. When the application starts the hardware
automatically switches to a more constrained environment: the
application mode.

The TKey hardware cores are memory mapped but the memory access is
different depending on mode. Firmware has complete access, except that
the Unique Device Secret (UDS) words are readable only once even in
firmware mode. The memory map is constrained when running in
application mode, e.g. FW\_RAM and UDS isn't readable, and several
other hardware addresses are either not readable or not writable for
the application.

When doing system calls from a device app the context switches back to
firmware mode. However, the UDS is still not available, protected by
two measures: 1) the UDS words can only be read out once and have
already been read by firmware when measuring the app, and, 2) the UDS
is protected by hardware after the execution leaves ROM for the first
time.

See the table in [the Developer
Handbook](https://dev.tillitis.se/memory/) for an overview about the
memory access control.

## Communication

The firmware communicates with the client using the
`UART_{RX,TX}_{STATUS,DATA}` registers. On top of that is uses three
protocols: The USB Mode protocol, the TKey framing protocol, and the
firmware's own protocol.

To communicate between the CPU and the CH552 USB controller it uses an
internal protocol, used only within the TKey, which we call the USB
Mode Protocol. It is used in both directions.

| *Name*   | *Size*    | *Comment*                          |
|----------|-----------|------------------------------------|
| Endpoint | 1B        | Origin or destination USB endpoint |
| Length   | 1B        | Number of bytes following          |
| Payload  | See above | Actual data from or to firmware    |

The different endpoints:

| *Name* | *Value* | *Comment*                                                            |
|--------|---------|----------------------------------------------------------------------|
| CCID   | 0x08    | USB CCID, a port for emulating a smart card                          |
| CH552  | 0x10    | USB controller control                                               |
| DEBUG  | 0x20    | A USB HID special debug pipe. Useful for debug prints.               |
| CDC    | 0x40    | USB CDC-ACM, a serial port on the client.                            |
| FIDO   | 0x80    | A USB FIDO security token device, useful for FIDO-type applications. |

You can turn on and off different endpoints dynamically by sending
commands to the `CH552` control endpoint. When the TKey starts only
the `CH552` and the `CDC` endpoints are active. To change this, send a
command to `CH552` in this form:

| *Name*   | *Size* | *Comment*                     |
|----------|--------|-------------------------------|
| Command  | 1B     | Command to the CH552 firmware |
| Argument | 1B     | Data for the command          |

Commands:

| *Name*           | *Value* | *Argument*           |
|------------------|---------|----------------------|
| Enable endpoints | 0x01    | Bitmask of endpoints |

On top of the USB Mode Protocol is [the TKey Framing
Protocol](https://dev.tillitis.se/protocol/) which is described in the
Developer Handbook.

The firmware uses a protocol on top of this framing layer which is
used to load a device application. All commands are initiated by the
client. All commands receive a reply. See [Firmware
protocol](http://dev.tillitis.se/protocol/#firmware-protocol) in the
Dev Handbook for specific details.

## Memory constraints

| *Name*  | *Size*    | *FW mode* | *App mode* |
|---------|-----------|-----------|------------|
| ROM     | 8 kByte   | r-x       | r          |
| FW\_RAM | 4 kByte*  | rw-       | -          |
| RAM     | 128 kByte | rwx       | rwx        |

* FW\_RAM is divided into the following areas:

- fw stack: 3000 bytes.
- resetinfo: 256 bytes.
- .data and .bss: 840 bytes.

## Firmware behaviour

The purpose of the firmware is to:

1. Load, measure, and start an application received from the client
   over the USB/UART or from one of two flash app slots.
2. Provide functionality to run only app's with specific BLAKE2s
   digests.
3. Provide system calls to access the filesystem and get other data.

The firmware binary is part of the FPGA bitstream as the initial
values of the Block RAMs used to construct the ROM. The ROM is located
at `0x0000_0000`. This is also the CPU reset vector.

### Reset type

When the TKey is started or resetted it can load an app from different
sources. We call this the reset type. Reset type is located in the
resetinfo part of FW\_RAM. The different reset types loads and start
an app from:

1. Flash slot 0 (default): `FLASH0` with a specific app hash defined
   in a constant in firmware.
2. Flash slot 1: `FLASH1`.
3. Flash slot 0 with a specific app hash left from previous app:
   `FLASH0_VER`
4. Flash slot 1 with a specific app hash left from previous app:
   `FLASH1_VER`.
5. Client: `CLIENT`.
6. Client with a specific app hash left from previous app:
   `CLIENT_VER`.

### Firmware state machine

Change of state occur when we receive specific I/O or a fatal error
occurs.

```mermaid
stateDiagram-v2
    S0: INITIAL
    S1: WAITCOMMAND
    S2: LOADING
    S3: LOAD_FLASH
    S4: LOAD_FLASH_MGMT
    S5: START
    SE: FAIL

    [*] --> S0

    S0 --> S1
    S0 --> S4: Default
    S0 --> S3

    S1 --> S1: Other commands
    S1 --> S2: LOAD_APP
    S1 --> SE: Error

    S2 --> S2: LOAD_APP_DATA
    S2 --> S5: Last block received
    S2 --> SE: Error

    S3 --> S5
    S3 --> SE

    S4 --> S5
    S4 --> SE

    SE --> [*]
    S5 --> [*]
```

States:

- *INITIAL*: Transitions to next state through reset type left in
  `FW_RAM`.
- *WAITCOMMAND*: Waiting for initial commands from client. Allows the
  commands `NAME_VERSION`, `GET_UDI`, `LOAD_APP`.
- *LOADING*: Expecting application data from client. Allows only the
  command `LOAD_APP_DATA` to continue loading the device app.
- *LOAD_FLASH*: Loading an app from flash. Allows no commands.
- *LOAD_FLASH_MGMT*: Loading and verifyiing a device app from flash.
  Allows no commands.
- *START*: Computes CDI. Possibly verifies app. Starts the
  application. Does not return to firmware. Allows no commands.
- *FAIL* - Halts CPU. Allows no commands.

Allowed data in state *INITIAL*:

| *reset type* | *next state*      |
|--------------|-------------------|
| `FLASH0`     | *LOAD_FLASH_MGMT* |
| `FLASH1`     | *LOAD_FLASH*      |
| `FLASH0_VER` | *LOAD_FLASH*      |
| `FLASH1_VER` | *LOAD_FLASH*      |
| `CLIENT`     | *WAITCOMMAND*     |
| `CLIENT_VER` | *WAITCOMMAND*     |

I/O in state *LOAD_FLASH*:

| *I/O*              | *next state* |
|--------------------|--------------|
| Last app data read | *START*      |

I/O in state *LOAD_FLASH_MGMT*:

| *I/O*              | *next state* |
|--------------------|--------------|
| Last app data read | *START*      |

Commands in state `waitcommand`:

| *command*             | *next state* |
|-----------------------|--------------|
| `FW_CMD_NAME_VERSION` | unchanged    |
| `FW_CMD_GET_UDI`      | unchanged    |
| `FW_CMD_LOAD_APP`     | *LOADING*    |

Commands in state `loading`:

| *command*              | *next state*                       |
|------------------------|------------------------------------|
| `FW_CMD_LOAD_APP_DATA` | unchanged or *START* on last chunk |

No other states allows commands.

See [Firmware protocol in the Dev
Handbook](http://dev.tillitis.se/protocol/#firmware-protocol) for the
definition of the specific commands and their responses.

Plain text explanation of the states:

- *INITIAL*: Start here. Check the `FW_RAM` for the `resetinfo` type
  for what to do next.

  For all types which begins with `FLASH_*`, set next state to
  *LOAD_FLASH*, otherwise set next state to *WAITCOMMAND*.

- *LOAD_FLASH*: Load device app from flash into RAM, app slot taken
  from context. Compute a BLAKE2s digest over the entire app.
  Transition to *START*.

- *LOAD_FLASH_MGMT*: Load device app from flash into RAM, app slot
  alway 0. Compute a BLAKE2s digest over the entire app. Register the
  app as a prospective management app if it later goes through
  verification. Transition to *START*.

- *WAITCOMMAND*: Wait for commands from the client. Transition to
  *LOADING* on `LOAD_APP` command, which also sets the size of the
  number of data blocks to expect.

- *LOADING*: Wait for several `LOAD_APP_DATA` commands until the last
  block is received, then transition to *START*.

- *START*: Compute the Compound Device Identifier (CDI). If we have a
  registered verification digest, verify that the app we are about to
  start is indeed the correct app.

  Clean up firmware data structures, enable the system calls, and
  start the app, which ends the firmware state machine. Hardware
  guarantees that we leave firmware mode automatically when the
  program counter leaves ROM.

- *FAIL*: Execute an illegal instruction which traps the CPU. Hardware
  detects a trapped CPU and blinks the status LED in red until power
  loss. No further instructions are executed.

After leaving *START* the device app is now running in application
mode. We can, however, return to firmware mode (excepting access to
the UDS) by doing system calls. Note that ROM is still readable, but
is now hardware protected from execution, except through the system
call mechanism.

If during this whole time any commands are received which are not
allowed in the current state, or any errors occur, we enter the *FAIL*
state.

### Golden path from start to default app

Firmware loads the device application at the start of RAM
(`0x4000_0000`) from either flash or from the client through the UART.
Firmware uses a part of the FW\_RAM for its own stack.

When reset is released, the CPU starts executing the firmware. It
begins in `start.S` by clearing all CPU registers, clears all FW\_RAM,
sets up a stack for itself there, and then jumps to `main()`. Also
included in the assembly part of firmware is an interrupt handler for
the system calls, but the handler is not yet enabled.

Beginning at `main()` it fills the entire RAM with pseudo random data
and setting up the RAM address and data hardware scrambling with
values from the True Random Number Generator (TRNG).

1. Check the special resetinfo area in FW\_RAM for reset type. Type
   zero means default behaviour, load from flash app slot 0, expecting
   the app there to have a specific hardcoded BLAKE2s digest.

2. Load app data from flash slot 0 into RAM.

3. Compute a BLAKE2s digest of the loaded app.

4. Compare the computed digest against the allowed app digest
   hardcoded in the firmware. If it's not equal, halt CPU.

7. [Start the device app](#start-the-device-app).

### Start the device app

1. Check if there is a verification digest left from the previous app
   in the resetinfo. If it is, compare with the loaded app's already
   computed digest. Halt CPU if different.

2. Compute the Compound Device Identifier
   ([CDI]((#compound-device-identifier-computation))) by doing a
   BLAKE2s using the Unique Device Secret (UDS), the application
   digest, and any User Supplied Secret (USS) digest already received.

3. Write the start address of the device app, currently `0x4000_0000`,
   to `APP_ADDR` and the size of the loaded binary to `APP_SIZE` to
   let the device application know where it is loaded and how large it
   is, if it wants to relocate in RAM.

4. Clear the stack part of `FW_RAM`.

5. Enable system call interrupt handler.

6. Start the application by jumping to the contents of `APP_ADDR`.
   Hardware automatically switch from firmware mode to application
   mode. In this mode some memory access is restricted, e.g. some
   addresses are inaccessible (`UDS`), and some are switched from
   read/write to read-only (see [the memory
   map](https://dev.tillitis.se/memory/)).

### Management app, chaining apps and verified boot

Normally, the TKey measures a device app and mixes it together with
the Unique Device Secret in hardware to produce the [Compound Device
Identifier]((#compound-device-identifier-computation)). The CDI can
then be used for creating key material. However, since any key
material created like this will change if the app is changed even the
slightest, this make it hard to upgrade apps and keep the key
material.

This is where a combination of measured boot and verified boot comes
in!

To support verified boot the firmware supports reset types with
verification. This means that the firmware will load an app as usual
either from flash or from the client, but before starting the app it
will verify the new app's computed digest with a verification digest.
The verification digest can either be stored in the firmware itself or
left to it from a previous app, a verified boot loader app.

Such a verified boot loader app:

- Might be loaded from either flash or client.

- Typically includes a security policy, for instance a public key and
  code to check a crytographic signature.

- Can be specifically trusted by firmware to be able to do filesystem
  management to be able to update an app slot on flash. Add the app's
  digest to `allowed_app_digest` in `mgmt_app.c` to allow it to allow
  it to use `PRELOAD_DELETE`, `PRELOAD_STORE`, and
  `PRELOAD_STORE_FIN`.

It works like this:

- The app reads a digest of the next app in the chain and the
  signature over the digest from either the filesystem (syscall
  `PRELOAD_GET_DIGSIG`) or sent from the client.

- If the signature provided over the digest is verified against the
  public key the app use the system call `RESET` with the reset type
  set to `START_FLASH0_VER`, `START_FLASH1_VER`, or `START_CLIENT_VER`
  depending on where it wants the next app to start from. It also
  sends the now verified app digest to the firmware in the same system
  call.

- The key is reset and firmware starts again. It checks:

  1. The reset type. Start from client or a slot in the filesystem?
  2. The expected digest of the next app.

- Firmware loads the app from the expected source.

- Firmware refuses to start if the loaded app has a different digest.

- If the app was allowed to start it can now use something
  deterministic left for it in resetinfo by the verified boot loader
  app as a seed for it's key material and no longer use CDI for the
  purpose.

We propose that a loader app can derive the seed for the next app by
creating a shared secret, perhaps something as easy as:

```
secret = blake2s(cdi, "name-of-next-app")
```

The loader shares the secret with the next app by putting it in the
part of `resetinfo` that is reserved for inter-app communication.

The next app can now use the secret as a seed for it's own key
material. Depending on the app's behaviour and the numer of keys it
needs it can derive more keys, for instance by having nonces stored on
its flash area and doing:

```
secret1 = blake2s(secret, nonce1)
secret2 = blake2s(secret, nonce2)
...
```

Now it can create many secrets deterministically, as long as there is
some space left on flash for the nonces and all of them can be traced
to the measured identity of the loader app, giving all the features of
the measured boot system.

### App loaded from client

The default is always to start from a verified app in flash slot
0. To be able to load an app from the client you have to send
something to the app to reset the TKey with a reset type of
`START_CLIENT` or `START_CLIENT_VER`.

After reset, firmware will:

1. Wait for data coming in through the UART.

2. The client sends the `FW_CMD_LOAD_APP` command with the size of
   the device app and the optional 32 byte hash of the user-supplied
   secret as arguments and gets a `FW_RSP_LOAD_APP` back. After
   using this it's not possible to restart the loading of an
   application.

3. On a sucessful response, the client will send multiple
   `FW_CMD_LOAD_APP_DATA` commands, together containing the full
   application.

4. On receiving`FW_CMD_LOAD_APP_DATA` commands the firmware places
   the data into `0x4000_0000` and upwards. The firmware replies
   with a `FW_RSP_LOAD_APP_DATA` response to the client for each
   received block except the last data block.

5. When the final block of the application image is received with a
   `FW_CMD_LOAD_APP_DATA`, the firmware measure the application by
   computing a BLAKE2s digest over the entire application. Then
   firmware send back the `FW_RSP_LOAD_APP_DATA_READY` response
   containing the digest.

6. [Start the device app](#start-the-device-app).

### User-supplied Secret (USS)

USS is a 32 bytes long secret provided by the user. Typically a client
program gets a secret from the user and then does a key derivation
function of some sort, for instance a BLAKE2s, to get 32 bytes which
it sends to the firmware to be part of the CDI computation.

### Compound Device Identifier computation

The CDI is computed by:

```
CDI = blake2s(UDS, blake2s(app), USS)
```

In an ideal world, software would never be able to read UDS at all and
we would have a BLAKE2s function in hardware that would be the only
thing able to read the UDS. Unfortunately, we couldn't fit a BLAKE2s
implementation in the FPGA.

The firmware instead does the CDI computation using the special
firmware-only `FW_RAM` which is invisible after switching to app mode.
We keep the entire firmware stack in `FW_RAM` and clear the stack just
before switching to app mode just in case.

We sleep for a random number of cycles before reading out the UDS,
call `blake2s_update()` with it and then immediately call
`blake2s_update()` again with the program digest, destroying the UDS
stored in the internal context buffer. UDS should now not be in
`FW_RAM` anymore. We can read UDS only once per power cycle so UDS
should now not be available even to firmware.

Then we continue with the CDI computation by updating with an optional
USS digest and finalizing the hash, storing the resulting digest in
`CDI`.

### System calls

The firmware provides a system call mechanism through the use of the
PicoRV32 interrupt handler. They are triggered by writing to the
trigger address: 0xe1000000. It's typically done with a function
signature like this:

```
int syscall(uint32_t number, uint32_t arg1, uint32_t arg2,
	    uint32_t arg3);
```

Arguments are system call number and up to 6 generic arguments passed
to the system call handler. The caller should place the system call
number in the a0 register and the arguments in registers a1 to a7
according to the RISC-V calling convention. The caller is responsible
for saving and restoring registers.

The syscall handler returns execution on the next instruction after
the store instruction to the trigger address. The return value from
the syscall is now available in x10 (a0).

The syscall numbers are kept in `syscall_num.h`. The syscalls are
handled in `syscall_handler()` in `syscall_handler.c`.

#### `RESET`

```
struct reset {
	uint32_t type;           // Reset type
	uint8_t app_digest[32];  // Digest of next app in chain to verify
	uint8_t next_app_data[220]; // Data to leave around for next app
};

struct reset rst;

syscall(TK1_SYSCALL_RESET, (uint32_t)&rst, 0, 0);
```

Resets the TKey. Does not return.

You can pass data to the firmware about the reset type `type` and a
digest that the next app must have. You can also leave some data to
the next app in the chain in `next_app_data`.

The types of the reset are defined in `resetinfo.h`:

| *Name*             | *Comment*                                      |
|--------------------|------------------------------------------------|
| `START_FLASH0`     | Load next app from flash slot 0                |
| `START_FLASH1`     | Load next app from flash slot 1                |
| `START_FLASH0_VER` | Load next app from flash slot 0, but verify it |
| `START_FLASH1_VER` | Load next app from flash slot 1, but verify it |
| `START_CLIENT`     | Load next app from client                      |
| `START_CLIENT_VER` | Load next app from client                      |

#### `ALLOC_AREA`

```
syscall(TK1_SYSCALL_ALLOC_AREA, 0, 0, 0);
```

Allocate a flash area for the current app. Returns 0 on success.

#### `DEALLOC_AREA`

```
syscall(TK1_SYSCALL_DEALLOC_AREA, 0, 0, 0);
```

Free an already allocated flash area for the current app. Returns 0 on
success.

#### `WRITE_DATA`

```
uint32_t offset = 0;
uint8_t buf[17];

TK1_SYSCALL_WRITE_DATA, offset, (uint32_t)buf, sizeof(buf))
```

Write data in `buf` to the app's flash area at byte `offset` within
the area. Returns 0 on success.

#### `READ_DATA`

```
uint32_t offset = 0;
uint8_t buf[17];

syscall(TK1_SYSCALL_READ_DATA, offset, (uint32_t)buf, sizeof(buf);
```

Read into `buf` at byte `offset` from the app's flash area.

#### `ERASE_DATA`

```
uint32_t offset = 0;
uint32_t size = 4096;

syscall(TK1_SYSCALL_ERASE_DATA, offset, size, 0);
```

Erase `size` bytes from `offset` within the area. Returns 0 on
success.

Both `size` and  `offset` must be a multiple of 4096 bytes.

#### `PRELOAD_DELETE`

```
syscall(TK1_SYSCALL_PRELOAD_DELETE, 0, 0, 0);
```

Delete the app in flash slot 1. Returns 0 on success. Only available
for the verified management app.

#### `PRELOAD_STORE`

```
uint8_t *appbinary;
uint32_t offset;
uint32_t size;

syscall(TK1_SYSCALL_PRELOAD_STORE, offset, (uint32_t)appbinary,
			  size);
```

Store an app, or possible just a block of an app, from the `appbinary`
buffer in flash slot 1 at byte `offset` If you can't find your entire
app in the buffer, call `PRELOAD_STORE` many times as you receive the
binary from the client. Returns 0 on success.

Only available for the verified management app.

#### `PRELOAD_STORE_FIN`

```
uint8_t app_digest[32];
uint8_t app_signature[64];
size_t app_size;

syscall(TK1_SYSCALL_PRELOAD_STORE_FIN, app_size,
		    (uint32_t)app_digest, (uint32_t)app_signature)
```

Finalize storing of an app where the complete binary size is
`app_size` in flash slot 1. Returns 0 on success. Only available for
the verified management app.

Compute the `app_digest` with BLAKE2s over the entire binary.

Sign `app_digest` with your Ed25519 private key and pass the
resulting signature in `app_signature`.

#### `PRELOAD_GET_DIGSIG`

```
uint8_t app_digest[32];
uint8_t app_signature[64];

syscall(TK1_SYSCALL_PRELOAD_GET_DIGSIG, (uint32_t)app_digest,
		(uint32_t)app_signature, 0);
```

Copies the digest and signature of app in flash slot 1 to `app_digest`
and `app_signature`. Returns 0 on success. Only available for the
verified management app.

#### `STATUS`

```
syscall(TK1_SYSCALL_PRELOAD_STATUS, 0, 0, 0);
```

Returns filesystem status. Non-zero when problems have been detected,
so far only that the first copy of the partition table didn't pass
checks.

#### `GET_VIDPID`

```
syscall(TK1_SYSCALL_PRELOAD_STATUS, 0, 0, 0);
```

Returns Vendor and Product ID. Notably the serial number is not
returned, so a device app can't identify what particular TKey it is
running on.

## Developing firmware

Standing in `hw/application_fpga/` you can run `make firmware.elf` to
build just the firmware. You don't need all the FPGA development
tools. See [the Developer Handbook](https://dev.tillitis.se/tools/)
for the tools you need. The easiest is probably to use our OCI image,
`ghcr.io/tillitis/tkey-builder`.

[Our version of qemu](https://dev.tillitis.se/tools/#qemu-emulator) is
also useful for debugging the firmware. You can attach GDB, use
breakpoints, et cetera.

There is a special make target for QEMU: `qemu_firmware.elf`, which
sets `-DQEMU_DEBUG`, so you can debug prints using the `debug_*()`
functions. Note that these functions are only usable in QEMU and that
you might need to `make clean` before building, if you have already
built before.

If you want debug prints to show up on the special TKey HID debug
endpoint instead, define `-DTKEY_DEBUG`. This might mean you can't fit
the firmware in the ROM space available, however. You will get a
warning if it doesn't fit. In that case, just use explicit
`puts(IO_DEBUG, ...)` or `puts(IO_CDC, ...)` and so on.

Note that if you use `TKEY_DEBUG` you *must* have something listening
on the corresponding HID device. It's usually the last HID device
created. On Linux, for instance, this means the last reported hidraw
in `dmesg` is the one you should do `cat /dev/hidrawX` on.

### tkey-libs

Most of the utility functions that the firmware use lives in
`tkey-libs`. The canonical place where you can find tkey-libs is at:

  https://github.com/tillitis/tkey-libs

but we have vendored it in for firmware use in `../tkey-libs`. See top
README for how to update.

### Preparing the filesystem

The TKey supports a simple filesystem. This filesystem must be
initiated before starting for the first time. You need a [TKey
Programmer Board](https://shop.tillitis.se/products/tkey-dev-kit) for
this part.

1. Choose your pre-loaded app. You /must/ have a pre-loaded app, for
   example `testloadapp`. Build it with the OCI image we use. The
   binary needs to produce the BLAKE2s digest in `allowed_app_digest`
   `tk1/mgmt_app.c`.

2. Write the filesystem to flash:

   ```
   $ cd ../tools
   $ ./load_preloaded_app.sh 0 ../fw/testloadapp/testloadapp.bin
   ```

If you want to use a different pre-loaded app you have to

1. Check the BLAKE2s digest of the app. You can use `tools/b2s` to
   compute it.

2. Update the `allowed_app_digest` in `tk1/mgmt_app.c`.

3. Create a new `default_partition.bin` using the
   `tools/partition_table`, typically:

   ```
   $ partition_table -app0 path/to/your/app.bin -o default_partition.bin
   ```

4. Flash the filesystem image:

  ```
  $ ./load_preloaded_app.sh 0 path/to/your/app.bin
  ```

### Test firmware

The test firmware is in `testfw`. It's currently a bit of a hack and
just runs through expected behavior of the hardware cores, giving
special focus to access control in firmware mode and application mode.

It outputs results on the UART. This means that you have to attach a
terminal program to the serial port device, even if it's running in
qemu. It waits for you to type a character before starting the tests.

It needs to be compiled with `-Os` instead of `-O2` in `CFLAGS` in the
ordinary `application_fpga/Makefile` to be able to fit in ROM.

### Test apps

There are a couple of test apps. All of them are controlled through
the USB CDC, typically by running picocom or similar terminal program,
like:

```
$ picocom /dev/ttyACM1
```

or similar.

- `fw/testapp`: Runs through a couple of tests that are now impossible
  to do in the `testfw`.
- `fw/reset_test`: Interactively test different reset scenarios.
- `fw/testloadapp`: Interactively test management app things like
  installing an app (hardcoded for a small happy blinking app, see
  `blink.h` for the entire binary!) and to test verified boot.