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

[Mikhail Rudenko]

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.

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