alexcekay opened a new pull request, #18787:
URL: https://github.com/apache/nuttx/pull/18787
## Summary
In sporadic cases it is possible that a SPI exchange returns stale RX data
from the DCACHE.
This occurs when:
- DCACHE is enabled
- DMA is used
The impact of this can be hard to debug and vanishes when the timing even
changes minimally.
This is caused by the DCACHE being invalidated before the actual DMA
transaction starts which violates the recommendations from AN4839 and also does
not match the implementation of other drivers of the STM32H7/STM32F7.
Fixed by invalidating the DCACHE right before the actual read, which matches
the implementation of the STM32F7 SPI driver.
## Testing
Tested on a Pixhawk v6x. Validated by setting a breakpoint in a driver
running on SPI3 that checks if the current RX data returned by a SPI transfer
matches the data format of another driver running on SPI3. Without the fix it
is visible that stale data is present that vanishes as soon as the cache is
invalidated.
```
$11 = {
spidev = {
ops = 0x81f05e0 <g_sp3iops>
},
spibase = 1073757184,
spiclock = 96000000,
spiirq = 67 'C',
rxresult = 176 '\260',
txresult = 128 '\200',
rxch = 573,
txch = 574,
rxbuf = 0x24015240 <g_spi3_rxbuf> "",
txbuf = 0x24015640 <g_spi3_txbuf> "\277",
buflen = 1024,
rxdma = 0x240007e0 <g_dmach+160>,
txdma = 0x240007f0 <g_dmach+176>,
rxsem = taken (0),
txsem = taken (0),
txccr = 1088,
rxccr = 1024,
initialized = true,
exclsem = available (1i),
frequency = 10000000,
actual = 6000000,
nbits = 8 '\b',
mode = 3 '\003',
config = FULL_DUPLEX
}
```
```
(gdb) x/8xw 0x24015240
0x24015240 <g_spi3_rxbuf>: 0x001c0000 0x04000a84 0x84055d00
0x00080012
0x24015250 <g_spi3_rxbuf+16>: 0x08840562 0x66000500
0x000e8405 0x055a0005
(gdb) call up_invalidate_dcache(0x24015240, 0x24015260)
(gdb) x/8xw 0x24015240
0x24015240 <g_spi3_rxbuf>: 0xdc002aff 0x9c0012ff 0x00000aff
0x2a009200
0x24015250 <g_spi3_rxbuf+16>: 0x9effee00 0xaa0008ff
0xd0fffeff 0x050064ff
```
After applying the fix the breakpoint is not hit anymore and the high-level
problem (gyroscope reading accelerometer data) does not happen anymore.
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