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

[Mikhail Rudenko <mike.rudenko@gmail.com>]

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/

>
>> [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