[PATCH 03/27] libcamera: dma_buf_allocator: Favour udmabuf over cma heap allocations

[Robert Mader]

On 25.04.25 17:07, Laurent Pinchart wrote:
> On Fri, Apr 25, 2025 at 09:01:33AM -0400, Nicolas Dufresne wrote:
>> Le jeudi 24 avril 2025 à 17:14 +0300, Laurent Pinchart a écrit :
>>> On Thu, Apr 24, 2025 at 02:51:42PM +0100, Bryan O'Donoghue wrote:
>>>> On 24/04/2025 14:17, Nicolas Dufresne wrote:
>>>>>> even if the CPU only 'reads' the data ?
>>>>> Its any CPU write -> GPU that in serious trouble on recent Intel. That
>>>>> being said, if nothing either flush or invalidate the cache, if you
>>>>> read before, write with GPU, and read again after that same buffer, the
>>>>> data you'll read may be from old cache. In practice, GPU don't do
>>>>> anything in-place, so that is non-issue here.
>>>> Well we have the GPU write to its own frame buffer and currently then
>>>> get a GBM pointer to that surface and memcpy() out into the libcamera
>>>> buffer be that CMA or UDMABuf.
>>>>
>>>> +	gbm_surface_lock_front_buffer(gbm_surface_);
>>>> +
>>>> +	sync.flags = DMA_BUF_SYNC_START | DMA_BUF_SYNC_READ;
>>>> +	ioctl(bo_fd_, DMA_BUF_IOCTL_SYNC, &sync);
>>>> +
>>>> +	if (data_len > framesize_) {
>>>> +		LOG(GBM, Error) << "Invalid read size " << data_len << " max is " << framesize_;
>>>> +		return -EINVAL;
>>>> +	}
>>>> +
>>>> +	memcpy(data, map_, data_len);
>>>> +
>>>> +	sync.flags = DMA_BUF_SYNC_END | DMA_BUF_SYNC_READ;
>>>> +	ioctl(bo_fd_, DMA_BUF_IOCTL_SYNC, &sync);
>>>> +
>>>> +	gbm_surface_release_buffer(gbm_surface_, gbm_bo_);
>>>> +
>>>>
>>>> That I believe should be cache-coherent for both CMA and UDMAbuf across
>>>> architectures.
>>>>
>>>> We had an interesting discussion on how render-to-texture would work -
>>>> if you created the render texture using eglCreateImageKHR.
>>>>
>>>> // render to texture
>>>>
>>>> dmabuf_handle = handle to CMA or UDMABuf buffer.
>>>>
>>>> EGLint attrs[] = {
>>>> 	EGL_DMA_BUF_PLANE0_FD_EXT, dmabuf_handle
>>>> };
>>>>
>>>> mytexture-output = glCreateImageKHR(display, context,
>>>>                                       EGL_LINUX_DMA_BUF_EXT,
>>>>                                       NULL, attrs);
>>>>
>>>> glBindFramebuffer(GL_FRAMBUFFER, someframebuffer);
>>>> glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0,
>>>>                          GL_TEXTURE_2D, mytexture-output);
>>>>
>>>>
>>>> if dmabuf_handle points to CMA heap - who is responsible to flush the
>>>> cache before the CPU accessess the content of the DMA buf handle..
>>>>
>>>> actually as I type that, I think the answer is that it is always the
>>>> responsibility of the CPU side to manage its own cache so..
>>>>
>>>> glDraw();
>>>>
>>>> sync.flags = DMA_BUF_SYNC_START | DMA_BUF_SYNC_READ;
>>>> ioctl(dmabuf_handle, DMA_BUF_IOCTL_SYNC, &sync);
>>>>
>>>> sync.flags = DMA_BUF_SYNC_END | DMA_BUF_SYNC_READ;
>>>> ioctl(dmabuf_handle, DMA_BUF_IOCTL_SYNC, &sync);
>>>>
>>>> something like that ..
>>>>
>>>>>> For GPU ISP - the only access the CPU should do is read the data to
>>>>>> generate some statistics. And even that - I would hope in the future
>>>>>> will be turned into operations performed by the GPU ...
>>>>> So read bayer, process in GPU, output YUV should just work. Thanks for
>>>>> clarification, running out of CMA is pretty common, defaults CMA
>>>>> reservation is usually very small. Appart from the buggy drivers,
>>>>> virtual memory is a much better choice, and this is mostly what this
>>>>> patch is about here.
>>>> Thinking about this patch.
>>>>
>>>> A user on a system such as imx/hantro is free to configure the system to
>>>> support CMA and UDMABuf.
>>>>
>>>> If you never pass that buffer to the video encoder - where the encoder
>>>> requires phys contig/cma heap memory - then you probably want udmabuf.
>>> Add the display controller here, it's not just the video encoder.
>>>
>>> Have you tried displaying images with cam (using the -D flag) on a
>>> system without an IOMMU, with this patch applied ? Does it still work ?
>> In this case glCreateImageKHR() is expected to fail once it discover
>> that the memory is scattered. In my experience, this part is well
>> handled.
> It will only fail if the GPU has no IOMMU, right ? What if the GPU has
> an IOMMU, but the display controller doesn't ?

This would be the RPi4 case (on the RPi5 the DC has an IOMMU). I've been 
testing this quite a bit in the context of 
https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8540 
- using udmabuf with software video decoders in order to reuse dmabuf 
fastpaths and avoid copies.

The DC in question supports DRM_FORMAT_YUV420, which is the native 
format used by most SW decoders (dav1d, ffpmeg, vpx, openh264, ...) for 
8-bit 420 content. So far the case has been handled perfectly as 
expected - import to GPU (including in Wayland compositors) works but 
KMS offloading fails in the test commit on the RPi4, while succeeding on 
RPi5.

I've also tested several other devices and have yet to hit a case where 
things blow up because some component requiring CMA tries to import an 
udmabuf. The concern that it could happen has been rased by RPi devs as 
well though 
(https://github.com/raspberrypi/linux/issues/6706#issuecomment-2706283848).

---

Either way, do we agree that the commit in question should have its own 
series for further discussion as it's only semi-related to the gpuISP?

>> What's missing sometimes is verification that memory
>> allocation have happened with CPU cached enabled. Without the mmu,
>> there is nothing to handle cached pages. Recent Intel graphics happily
>> succeed, resulting in a funny mess on screen.
>>
>> I assume the code here is just for example, and that there is failure
>> and fallback in place.
>>
>>>> The reverse is also true.
>>>>
>>>> Its almost use-case specific. If you want the encoder you need CMA and
>>>> if you just want to say - display the output in cheese you want UDMAbuf.
>>>>
>>>> Probably the right thing to do is to leave CMA heap as default and leave
>>>> it to the system designer to configure the system as they wish.
>>>>
>>>> In my case, switching off the CMA heap or perhaps a libcamera config/env
>>>> variable to specify which to use...
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.libcamera.org/pipermail/libcamera-devel/attachments/20250425/060c9329/attachment.htm>