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

[Nicolas Dufresne <nicolas@ndufresne.ca>]

Le jeudi 27 février 2025 à 23:46 +0300, Mikhail Rudenko a écrit :
> Hi Jacopo,
> 
> On 2025-02-27 at 18:05 +01, Jacopo Mondi <jacopo.mondi@ideasonboard.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@ideasonboard.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.
> > 
> > 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.
> 
> Meanwhile, I have posted a patch, which re-enables cache management ops
> for non-coherent dmabufs exported from dma-contig allocator [1]. It is
> currently waiting for review.
> 
> [1] https://lore.kernel.org/all/20250128-b4-rkisp-noncoherent-v3-1-baf39c997d2a@gmail.com/

Thanks for you work, this matches what I was referring to missing in my
previous reply.

I don't think there is any intention to block or deprecate it, but
partially enabling leads to problems. Do we need something in the SG
allocator to ?

Nicolas

> 
> > 
> > > [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
>