arch/arm: fix DMA transfers with sizes smaller than 4 bytes.#19206
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When the buffer to send has a size not multiple of 4 bytes, 4-byte words are sent correctly, but when the code reaches the section to send the remaining bytes (1, 2 or 3) it was taking the remaining byte count as origin of the copy, instead of the buffer (as it should). This commit fixes it to correctly copy from the buffer pointer. Signed-off-by: Carlos Sanchez <carlossanchez@geotab.com>
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Summary
All STM32 SDIO/SDMMC drivers are affected by this bug which comes from the original addition of interrupt handing to the base STM32 SDIO driver in 2009 (373959f).
When the buffer to send has a size not multiple of 4 bytes, 4-byte words are sent correctly, but when the code reaches the section to send the remaining bytes (1, 2 or 3) it takes the remaining byte count as origin of the copy, instead of the buffer (as it should). The explicit cast (used to convert a uint32_t pointer to a uint8_t pointer) prevents the compiler warning/error to appear and hides the fact an integer (non-pointer) is used as a pointer.
Impact
This has been around since 2009 and has not triggered, probably because most (all?) SDIO transfers are word-sized so that section of the code is not being used at all.
Testing
The change has been tested on custom HW, using STM32H5, with STM32H7 SDMMC driver ported (it requires almost no changes to work on STM32H5). This has no effect (meaning: the driver still works after this change). I have not been able to find a use case triggering the bug, so I can not really test that this fixes it.