linux-kernel - Re: [PATCH] media: rkisp1: allow non-coherent video capture buffers

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Message-ID: <CAAFQd5DS2x3sX59EdkTEMuO=eXc=Q0jD44EUwMisP6ocWz11qg@mail.gmail.com>
Date: Fri, 28 Feb 2025 19:00:57 +0900
From: Tomasz Figa <tfiga@...omium.org>
To: Jacopo Mondi <jacopo.mondi@...asonboard.com>
Cc: Mikhail Rudenko <mike.rudenko@...il.com>, 
	Laurent Pinchart <laurent.pinchart@...asonboard.com>, Dafna Hirschfeld <dafna@...tmail.com>, 
	Mauro Carvalho Chehab <mchehab@...nel.org>, Heiko Stuebner <heiko@...ech.de>, linux-media@...r.kernel.org, 
	linux-rockchip@...ts.infradead.org, linux-arm-kernel@...ts.infradead.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] media: rkisp1: allow non-coherent video capture buffers

Hi Jacopo,

On Fri, Feb 28, 2025 at 2:11 AM Jacopo Mondi
<jacopo.mondi@...asonboard.com> wrote:
>
> Hi Mikhail
>
> On Tue, Jan 14, 2025 at 07:00:39PM +0300, Mikhail Rudenko wrote:
> >
> > Hi Laurent,
> >
> > On 2025-01-03 at 17:23 +02, Laurent Pinchart <laurent.pinchart@...asonboard.com> wrote:
> >
> > > On Thu, Jan 02, 2025 at 06:35:00PM +0300, Mikhail Rudenko wrote:
> > >> Currently, the rkisp1 driver always uses coherent DMA allocations for
> > >> video capture buffers. However, on some platforms, using non-coherent
> > >> buffers can improve performance, especially when CPU processing of
> > >> MMAP'ed video buffers is required.
> > >>
> > >> For example, on the Rockchip RK3399 running at maximum CPU frequency,
> > >> the time to memcpy a frame from a 1280x720 XRGB32 MMAP'ed buffer to a
> > >> malloc'ed userspace buffer decreases from 7.7 ms to 1.1 ms when using
> > >> non-coherent DMA allocation. CPU usage also decreases accordingly.
> > >
> > > What's the time taken by the cache management operations ?
> >
> > Sorry for the late reply, your question turned out a little more
> > interesting than I expected initially. :)
> >
> > When capturing using Yavta with MMAP buffers under the conditions mentioned
> > in the commit message, ftrace gives 437.6 +- 1.1 us for
> > dma_sync_sgtable_for_cpu and 409 +- 14 us for
> > dma_sync_sgtable_for_device. Thus, it looks like using non-coherent
> > buffers in this case is more CPU-efficient even when considering cache
> > management overhead.
> >
> > When trying to do the same measurements with libcamera, I failed. In a
> > typical libcamera use case when MMAP buffers are allocated from a
> > device, exported as dmabufs and then used for capture on the same device
> > with DMABUF memory type, cache management in kernel is skipped [1]
> > [2]. Also, vb2_dc_dmabuf_ops_{begin,end}_cpu_access are no-ops [3], so
> > DMA_BUF_IOCTL_SYNC from userspace does not work either.
> >
> > So it looks like to make this change really useful, the above issue of
> > cache management for libcamera/DMABUF/videobuf2-dma-contig has to be
> > solved. I'm not an expert in this area, so any advice is kindly welcome. :)
>
> It would be shame if we let this discussion drop dead.. cache
> management policies are relevant for performances, specifically for
> cpu access, and your above 7.7ms vs 1.1 ms test clearly shows that.
>
> >
> > [1] https://git.linuxtv.org/media.git/tree/drivers/media/common/videobuf2/videobuf2-core.c?id=94794b5ce4d90ab134b0b101a02fddf6e74c437d#n411
>
> I would like to know from Hans if the decision to disallow cache-hints
> for dmabuf importers is a design choice or is deeply rooted in other
> reasons I might be missing.

When DMA-buf is used, the responsibility for cache management is
solely on the CPU users' side, so cache-hints don't really apply. It's
the exporter (=allocator) who determines the mapping policy of the
buffer and provides necessary DMA_BUF_SYNC operations (can be no-op if
the buffer is coherent).

Best regards,
Tomasz

>
> I'm asking because the idea is for libcamera to act solely as dma-buf
> importer, the current alloc-export-then-import trick is an helper for
> applications to work around the absence of a system allocator.
>
> If the requirement to disable cache-hints for importers cannot be
> lifted, for libcamera it means we would not be able to use it.
>
>
> > [2] https://git.linuxtv.org/media.git/tree/drivers/media/common/videobuf2/videobuf2-core.c?id=94794b5ce4d90ab134b0b101a02fddf6e74c437d#n829
> > [3] https://git.linuxtv.org/media.git/tree/drivers/media/common/videobuf2/videobuf2-dma-contig.c?id=94794b5ce4d90ab134b0b101a02fddf6e74c437d#n426
> >
> > --
> > Best regards,
> > Mikhail Rudenko
> >
>