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

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Date:   Sun, 5 Aug 2018 01:41:16 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     "Matwey V. Kornilov" <matwey@....msu.ru>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Tomasz Figa <tfiga@...omium.org>,
        Ezequiel Garcia <ezequiel@...labora.com>,
        Hans de Goede <hdegoede@...hat.com>,
        Hans Verkuil <hverkuil@...all.nl>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>, mingo@...hat.com,
        Mike Isely <isely@...ox.com>,
        Bhumika Goyal <bhumirks@...il.com>,
        Colin King <colin.king@...onical.com>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Kieran Bingham <kieran.bingham@...asonboard.com>,
        keiichiw@...omium.org
Subject: Re: [PATCH 2/2] media: usb: pwc: Don't use coherent DMA buffers for
 ISO transfer

On Sun, Aug 05, 2018 at 11:33:38AM +0300, Matwey V. Kornilov wrote:
> >> Taken together, those measurements look like a pretty good argument for
> >> always using dma_sync_single_for_cpu in the driver.  Provided results
> >> on other platforms aren't too far out of line with these results.
> >
> > Logically speaking on no-mmio no-swiotlb platforms dma_sync_single_for_cpu
> > and dma_unmap should always be identical.  With the migration towards
> > everyone using dma-direct and dma-noncoherent this is actually going to
> > be enforced, and I plan to move that enforcement to common code in the
> > next merge window or two.
> >
> 
> I think that Alan means that using dma_sync_single_for_cpu() we save
> time required for subsequent dma_map() call (which is required when we
> do dma_unmap()).

The point still stands.  By definition for a correct DMA API
implementation a dma_sync_single_for_cpu/dma_sync_single_for_device
pair is always going to be cheaper than a dma_unmap/dma_map pair,
although for many cases the difference might be so small that it is
not measureable.

If you reuse a buffer using dma_sync_single* is always the right thing
to do vs unmapping and remapping it.