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

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Date:   Mon, 23 Jul 2018 14:57:23 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     "Matwey V. Kornilov" <matwey@....msu.ru>
cc:     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>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        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 Mon, 23 Jul 2018, Matwey V. Kornilov wrote:

> I've tried to strategies:
> 
> 1) Use dma_unmap and dma_map inside the handler (I suppose this is
> similar to how USB core does when there is no URB_NO_TRANSFER_DMA_MAP)

Yes.

> 2) Use sync_cpu and sync_device inside the handler (and dma_map only
> once at memory allocation)
> 
> It is interesting that dma_unmap/dma_map pair leads to the lower
> overhead (+1us) than sync_cpu/sync_device (+2us) at x86_64 platform.
> At armv7l platform using dma_unmap/dma_map  leads to ~50 usec in the
> handler, and sync_cpu/sync_device - ~65 usec.
> 
> However, I am not sure is it mandatory to call
> dma_sync_single_for_device for FROM_DEVICE direction?

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.

Alan Stern