Re: [PATCH v2] crypto: atmel-tdes - sync output bounce buffer before DMA

[Thorsten Blum <blum@kernel.org>]

On Wed, Sep 02, 2026 at 09:23:23PM +0200, Karl Mehltretter wrote:
> The slow path maps its output bounce buffer once at probe time with
> dma_map_single() and DMA_FROM_DEVICE, then reuses the mapping for every
> request.  After the CPU copies a result from the buffer,
> dma_sync_single_for_device() must hand the buffer back to the device before
> the next DMA transfer.  The driver omits this call, so cache lines from the
> previous result can remain valid while the device writes the next one.
> 
> This bug was masked by the completion paths calling
> dma_sync_single_for_device() immediately before the CPU copied the output,
> where dma_sync_single_for_cpu() was required.  For DMA_FROM_DEVICE on
> ARM926, dma_sync_single_for_device() invokes arm926_dma_map_area(), which
> invalidates the cache lines.  The misplaced call therefore discarded the
> stale lines before every copy-out.
> 
> Commit c8a9a647532f ("crypto: atmel-tdes - fix DMA sync direction")
> correctly changed the completion paths to call dma_sync_single_for_cpu().
> On ARM926, that function invokes arm926_dma_unmap_area(), which is a no-op.
> The missing pre-DMA dma_sync_single_for_device() was therefore exposed on
> ARM926-based SAM9X60 and SAM9X7 SoCs.
> 
> With CONFIG_CRYPTO_SELFTESTS=y all four DES/TDES algorithms fail on
> SAM9X75:
> 
>   alg: skcipher: atmel-ecb-tdes encryption test failed (wrong result) on
>   test vector 2, cfg="unaligned buffer, offset=1"
> 
> Call dma_sync_single_for_device() for the output buffer before starting DMA
> in both atmel_tdes_crypt_pdc() and atmel_tdes_crypt_dma().
> 
> Fixes: c8a9a647532f ("crypto: atmel-tdes - fix DMA sync direction")
> Cc: stable@vger.kernel.org
> Assisted-by: LLM
> Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
> ---
> Changes in v2:
> - Reword the changelog and use full function names. No code changes.
>   (Thorsten)
> 
> Link to v1:
>   https://lore.kernel.org/r/20260829045316.92931-1-kmehltretter@gmail.com/
> 
> Tested on top of:
> 
>   crypto: atmel-tdes - zero-initialize device state
>   https://lore.kernel.org/r/20260829035821.67220-1-kmehltretter@gmail.com/
> 
> Without that fix, on the tested SAM9X75 the DES/TDES self-tests hang on
> their first requests before reaching this test vector, so the failure
> fixed here is not observable on an otherwise unpatched tree.
> 
> The two patches are independent and apply in either order.
> 
>  drivers/crypto/atmel-tdes.c | 4 ++++
>  1 file changed, 4 insertions(+)

LGTM, thanks.

Reviewed-by: Thorsten Blum <blum@kernel.org>