Re: [PATCH 06/21] powerpc: dma-mapping: minimize for_cpu flushing

Arnd Bergmann
2023-03-27T13:02:50Z

On Mon, Mar 27, 2023, at 14:56, Christophe Leroy wrote:
> Le 27/03/2023 à 14:13, Arnd Bergmann a écrit :
>> From: Arnd Bergmann <arnd@arndb.de>
>>
>> The powerpc dma_sync_*_for_cpu() variants do more flushes than on other
>> architectures. Reduce it to what everyone else does:
>>
>> - No flush is needed after data has been sent to a device
>>
>> - When data has been received from a device, the cache only needs to
>> be invalidated to clear out cache lines that were speculatively
>> prefetched.
>>
>> In particular, the second flushing of partial cache lines of bidirectional
>> buffers is actively harmful -- if a single cache line is written by both
>> the CPU and the device, flushing it again does not maintain coherency
>> but instead overwrite the data that was just received from the device.
>
> Hum ..... Who is right ?
>
> That behaviour was introduced by commit 03d70617b8a7 ("powerpc: Prevent
> memory corruption due to cache invalidation of unaligned DMA buffer")
>
> I think your commit log should explain why that commit was wrong, and
> maybe say that your patch is a revert of that commit ?

Ok, I'll try to explain this better. To clarify here: the __dma_sync()
function in commit 03d70617b8a7 is used both before and after a DMA,
but my patch 05/21 splits this in two, and patch 06/21 only changes
the part that gets called after the DMA-from-device but leaves the
part before DMA-from-device unchanged, which Andrew's patch
addressed.

As I mentioned in the cover letter, it is still unclear whether
we want to consider this the expected behavior as the documentation
seems unclear, but my series does not attempt to answer that
question.

Arnd