This commit uses wrapping_sub for subtraction in update_credit.
The motivation for this is that currently the rpi-pico-w example panics
(at least for me) with the following error:
3.825277 INFO init done
└─ cyw43::{impl#4}::init::{async_fn#0} @ /embassy/cyw43/src/fmt.rs:138
3.825486 INFO Downloading CLM...
└─ cyw43::{impl#2}::init::{async_fn#0} @ /embassy/cyw43/src/fmt.rs:138
3.841328 WARN TX stalled
└─ cyw43::{impl#4}::run::{async_fn#0} @ /embassy/cyw43/src/fmt.rs:151
3.845549 ERROR panicked at 'attempt to subtract with overflow', /embassy/cyw43/src/lib.rs:919:16
└─ panic_probe::print_defmt::print @ .cargo/registry/src/github.com-1ecc6299db9ec823/panic-probe-0.3.0/src/lib.rs:91
────────────────────────────────────────────────────────────────────────────────
stack backtrace:
0: HardFaultTrampoline
<exception entry>
1: lib::inline::__udf
at ./asm/inline.rs:181:5
2: __udf
at ./asm/lib.rs:51:17
3: cortex_m::asm::udf
at .cargo/registry/src/github.com-1ecc6299db9ec823/cortex-m-0.7.6/src/asm.rs:43:5
4: rust_begin_unwind
at .cargo/registry/src/github.com-1ecc6299db9ec823/panic-probe-0.3.0/src/lib.rs:72:9
5: core::panicking::panic_fmt
at rustc/1c7b36d4db582cb47513a6c7176baaec1c3346ab/library/core/src/panicking.rs:142:14
6: core::panicking::panic
at /rustc/1c7b36d4db582cb47513a6c7176baaec1c3346ab/library/core/src/panicking.rs:48:5
7: cyw43::Runner<PWR,SPI>::update_credit
at /embassy/cyw43/src/lib.rs:919:16
8: cyw43::Runner<PWR,SPI>::rx
at /embassy/cyw43/src/lib.rs:808:9
9: cyw43::Runner<PWR,SPI>::run::{{closure}}
at /embassy/cyw43/src/lib.rs:727:21
10: <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll
at /rustc/1c7b36d4db582cb47513a6c7176baaec1c3346ab/library/core/src/future/mod.rs:91:19
11: cyw43_example_rpi_pico_w::__wifi_task_task::{{closure}}
at src/main.rs:32:17
total_len is already rounded up, so the `+ 3` is not needed.
And even if it was, the calculation should have been `((total_len + 3) / 4)`.
`(total_len + 3 / 4)` is equivalent to `total_len` and can overflow
the slice, leading to a panic which can easily be triggered by sending
large ICMP ECHO packets to the device.
This commit adds two constants which are intended to be used for setting
the `Word Length` and `High Speed` fields in the gSPR register
(address: 0x0000, bit: 0 and bit 4).
Currently, this field is being set by the following line:
```rust
// 32bit, little endian.
self.write32_swapped(REG_BUS_CTRL, 0x00010031).await;
```
Assuming that we are sending these bits using the gSPI write protocol
and using 16-bit word operation in little endian (which I think might
be the default) then the data bytes should be packed like this:
```
+--+--+--+--+
|D1|D0|D3|D2|
+--+--+--+--+
val (hex): 0x00010031
val (bin): 00000000000000010000000000110001
rotated(16): 00000000001100010000000000000001
```
If we split val into bytes and rotated the bits we get:
```
Split into bytes:
D3 D2 D1 D0
00000000 00000001 00000000 00110001
Rotate 16 and split into bytes:
D1 D0 D3 D2
00000000 00110001 00000000 00000001
```
Looking at the write procotol it seems to me that the above will
indeed set the `Word Length` to 1 but will also set other values.
```
Status enable (1=default)
D1 D0 D3 D2 ↓
00000000 00110001 00000000 00000001
↑↑ ↑↑ ↑
|| |Word Length (1=32-bit)
|| |
|| Endianess (0=Little)
||
|High-speed mode (1=High speed (default))
|
Interrupt polarity (1=high (default))
```
This commit suggests adding the above mentioned constants for setting
the only the word length field and the high speed field.
This commit renames the REG_BUS_FEEDBEAD to REG_BUS_TEST_RO
(Read-Only) which is the name used in the specification, section 4.2.3
Table 6.
It also adds a constant named REG_BUS_TEST_RW (Read-Write) to represent
the dummy register which the host can use to write data and read back
to check that the gSPI interface is working properly.
This commit add comments about what CLM stands for.
The motivation of this is that I think it helps understanding the code
for users who are new to the codebase (like me).
For some reason I got strange events on channel 1 (ASYNCEVENT_HEADER):
0.647329 WARN unexpected ehternet type 0x0508, expected Qualcom ether type 0x886c
This patch improves the validation of BCD WHD events to minimize the
risk for panic.
