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From:   "Matwey V. Kornilov" <matwey@sai.msu.ru>
Date:   Tue, 24 Jul 2018 21:56:09 +0300
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Subject: Re: [PATCH 2/2] media: usb: pwc: Don't use coherent DMA buffers for
 ISO transfer
To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     Tomasz Figa <tfiga@chromium.org>,
        Ezequiel Garcia <ezequiel@collabora.com>,
        Hans de Goede <hdegoede@redhat.com>,
        Hans Verkuil <hverkuil@xs4all.nl>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        Steven Rostedt <rostedt@goodmis.org>, mingo@redhat.com,
        Mike Isely <isely@pobox.com>,
        Bhumika Goyal <bhumirks@gmail.com>,
        Colin King <colin.king@canonical.com>,
        Linux Media Mailing List <linux-media@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Kieran Bingham <kieran.bingham@ideasonboard.com>,
        keiichiw@chromium.org
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2018-07-23 21:57 GMT+03:00 Alan Stern <stern@rowland.harvard.edu>:
> 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.

Well, I measured the following at armv7l. The handler execution time
(URB_NO_TRANSFER_DMA_MAP is used for all cases):

1) coherent DMA: ~3000 usec (pwc is not functional)
2) explicit dma_unmap and dma_map in the handler: ~52 usec
3) explicit dma_sync_single_for_cpu (no dma_sync_single_for_device): ~56 usec

So, I suppose that unfortunately Tomasz suggestion doesn't work. There
is no performance improvement when dma_sync_single is used.

At x86_64 the following happens:

1) coherent DMA: ~2 usec
2) explicit dma_unmap and dma_map in the handler: ~3.5 usec
3) explicit dma_sync_single_for_cpu (no dma_sync_single_for_device): ~4 usec

So, whats to do next? Personally, I think that DMA streaming API
introduces not so great overhead.
Does anybody happy with turning to streaming DMA or I'll introduce
module-level switch as Ezequiel suggested?


>
> Alan Stern
>


-- 
With best regards,
Matwey V. Kornilov.
Sternberg Astronomical Institute, Lomonosov Moscow State University, Russia
119234, Moscow, Universitetsky pr-k 13, +7 (495) 9392382