Re: [PATCH v4 2/2] media: usb: pwc: Don't use coherent DMA buffers for ISO transfer

[Matwey V. Kornilov]

2018-08-17 20:44 GMT+03:00 Matwey V. Kornilov <matwey.kornilov@xxxxxxxxx>:
> ÐÑ, 10 ÐÐÐ. 2018 Ð. Ð 17:27, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>:
>>
>> On Fri, 10 Aug 2018, Laurent Pinchart wrote:
>>
>> > > > Aren't you're missing a dma_sync_single_for_device() call before
>> > > > submitting the URB ? IIRC that's required for correct operation of the DMA
>> > > > mapping API on some platforms, depending on the cache architecture. The
>> > > > additional sync can affect performances, so it would be useful to re-run
>> > > > the perf test.
>> > >
>> > > This was already discussed:
>> > >
>> > > https://lkml.org/lkml/2018/7/23/1051
>> > >
>> > > I rely on Alan's reply:
>> > >
>> > > > According to Documentation/DMA-API-HOWTO.txt, the CPU should not write
>> > > > to a DMA_FROM_DEVICE-mapped area, so dma_sync_single_for_device() is
>> > > > not needed.
>> >
>> > I fully agree that the CPU should not write to the buffer. However, I think
>> > the sync call is still needed. It's been a long time since I touched this
>> > area, but IIRC, some cache architectures (VIVT ?) require both cache clean
>> > before the transfer and cache invalidation after the transfer. On platforms
>> > where no cache management operation is needed before the transfer in the
>> > DMA_FROM_DEVICE direction, the dma_sync_*_for_device() calls should be no-ops
>> > (and if they're not, it's a bug of the DMA mapping implementation).
>>
>> In general, I agree that the cache has to be clean before a transfer
>> starts.  This means some sort of mapping operation (like
>> dma_sync_*_for-device) is indeed required at some point between the
>> allocation and the first transfer.
>>
>> For subsequent transfers, however, the cache is already clean and it
>> will remain clean because the CPU will not do any writes to the buffer.
>> (Note: clean != empty.  Rather, clean == !dirty.)  Therefore transfers
>> following the first should not need any dma_sync_*_for_device.
>>
>> If you don't accept this reasoning then you should ask the people who
>> wrote DMA-API-HOWTO.txt.  They certainly will know more about this
>> issue than I do.
>
> Laurent,
>
> I have not seen any data corruption or glitches without
> dma_sync_single_for_device() on ARM and x86.
> It takes additional ~20usec for dma_sync_single_for_device() to run on
> ARM. So It would be great to ensure that we can reliable save another
> 15% running time.

DMA-API-HOWTO.txt has and example with my_card_interrupt_handler()
function where it says that "CPU should not write to
DMA_FROM_DEVICE-mapped area, so dma_sync_single_for_device() is not
needed here."

I've found that, for instance, drivers/crypto/caam/caamrng.c follows
DMA-API-HOWTO.txt and don't use dma_sync_single_for_device() in the
same case as we have in pwc.

>
>>
>> Alan Stern
>>
>
>
> --
> With best regards,
> Matwey V. Kornilov



-- 
With best regards,
Matwey V. Kornilov