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

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Message-ID: <CAAFQd5ACseSBg4WZnUnbtwh+i63xcKMBFKnfc4_Aqfmnz0s=7A@mail.gmail.com>
Date: Wed, 15 Jan 2025 17:31:42 +0900
From: Tomasz Figa <tfiga@...omium.org>
To: Mikhail Rudenko <mike.rudenko@...il.com>
Cc: 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 Mikhail and Laurent,

On Wed, Jan 15, 2025 at 2:07 AM Mikhail Rudenko <mike.rudenko@...il.com> 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.

Oops, so I believe this is a bug. When an MMAP buffer is allocated in
the non-coherent mode, those ops should perform proper cache
maintenance.

Let me send a patch to fix this in a couple of days unless someone
does it earlier.

Best regards,
Tomasz

>
> 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. :)
>
> [1] https://git.linuxtv.org/media.git/tree/drivers/media/common/videobuf2/videobuf2-core.c?id=94794b5ce4d90ab134b0b101a02fddf6e74c437d#n411
> [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
>