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

[Ezequiel Garcia]

Alan, Laurent:

On Fri, 2018-08-10 at 10:27 -0400, Alan Stern wrote:
> 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.
> 

Can either of you ack or nack this change? I'd like to see this merged,
or either re-worked, so we can merge it.

Thanks!
Eze