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

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Date:   Mon, 30 Jul 2018 13:14:32 +0900
From:   Tomasz Figa <tfiga@...omium.org>
To:     "Matwey V. Kornilov" <matwey@....msu.ru>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        Ezequiel Garcia <ezequiel@...labora.com>, hdegoede@...hat.com,
        Hans Verkuil <hverkuil@...all.nl>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        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 Wed, Jul 25, 2018 at 10:47 PM Matwey V. Kornilov <matwey@....msu.ru> wrote:
>
> 2018-07-24 23:55 GMT+03:00 Alan Stern <stern@...land.harvard.edu>:
> > On Tue, 24 Jul 2018, Matwey V. Kornilov wrote:
> >
> >> 2018-07-23 21:57 GMT+03:00 Alan Stern <stern@...land.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

That's very strange because on ARM dma_unmap_single() does exactly the
same thing as dma_sync_single_for_cpu():

arm_dma_map_page()
https://elixir.bootlin.com/linux/v4.18-rc7/source/arch/arm/mm/dma-mapping.c#L129
arm_dma_unmap_page()
https://elixir.bootlin.com/linux/v4.18-rc7/source/arch/arm/mm/dma-mapping.c#L159

arm_dma_sync_single_for_cpu()
https://elixir.bootlin.com/linux/v4.18-rc7/source/arch/arm/mm/dma-mapping.c#L167

Could you post the code you used for testing of cases 2) and 3)?

Best regards,
Tomasz