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

[Tomasz Figa]

On Wed, Jan 15, 2025 at 10:30 PM Mikhail Rudenko <mike.rudenko@xxxxxxxxx> wrote:
>
> Hi Tomasz,
>
> On 2025-01-15 at 17:31 +09, Tomasz Figa <tfiga@xxxxxxxxxxxx> wrote:
>
> > Hi Mikhail and Laurent,
> >
> > On Wed, Jan 15, 2025 at 2:07 AM Mikhail Rudenko <mike.rudenko@xxxxxxxxx> wrote:
> >>
> >>
> >> Hi Laurent,
> >>
> >> On 2025-01-03 at 17:23 +02, Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx> 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.
>
> Thanks for pointing this out!
>
> > Let me send a patch to fix this in a couple of days unless someone
> > does it earlier.
>
> Now that we know that this is a bug, not an API misuse from my side, I
> can fix this myself and send a v2. Would this be okay for you?

I'd be more than happy :)

>
> > 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
> >>
>
>
> --
> Best regards,
> Mikhail Rudenko