Interest in Contributing to VKMS Development

[Maíra Canal]

Hi Srikar & Louis,

On 01/12/24 02:36, Louis Chauvet wrote:
> +Cc: dri-devel at lists.freedesktop.org
> 
> On 30/11/24 - 13:49, Ananta Srikar Puranam wrote:
>> Dear Louis Chauvet,
> 
> Hi Srikar!
> 
>> I was able to successfully compile Linux with VKMS enabled and have set up
>> igt-gpu-tests in a QEMU machine. I can now run some tests successfully.
> 
> Congratulation!
>   
>> I have a surface-level understanding of the DRM and KMS subsystems
>> (framebuffers, planes, CRTCs, encoders, and connectors) and am eager to dive
>> deeper into the VKMS module and contribute.
> 
> I am very happy to see some interest in VKMS development, do you have a
> specific feature that you want to test?
> 
>> I understand VKMS helps test the DRM subsystem, but I’m struggling to fully
>> grasp how the absence of visible display output aligns with its testing
>> goals. Any guidance on this would be highly appreciated.
> 
> That the whole point of "V"KMS, everything is virtual, even the display
> output :)
> 
> There are two way to get some information about the output content:
> - CRC compuation [1]
> - Writeback connector [2]
> 
> The CRC will allows you to write a test like this:
> 
> 	create_reference_image();
> 	ref_crc = grab_crc();
> 
> 	test_procedure();
> 	test_crc = grab_crc();
> 
> 	assert(ref_crc, test_crc);
> 
> This is used in the igt test "kms_plane" to test color conversion, they
> compute a reference CRC and then test the conversion between multiple
> formats [3].
> 
> The writeback connector is useful to grab a "screenshot" of the output,
> but it is a bit less efficient. It can be used when a simple "reference
> CRC" is not possible.
> To grab a picture from the writeback connector, you have to do something
> like this:
> 
> 	setup_pipeline();
> 	wb_fb = create_fb();
> 	while (true) {
> 		send_fb_to_wb_connector(wb_fb);
> 		wait_for_picture();
> 		do_something_with_result(wb_fb);
> 	}
> 
> There is a writeback example in igt [4], but I don't know how it works.
> 
> [1]:https://elixir.bootlin.com/linux/v6.12.1/source/drivers/gpu/drm/vkms/vkms_composer.c#L209
> [2]:https://elixir.bootlin.com/linux/v6.12.1/source/drivers/gpu/drm/vkms/vkms_writeback.c#L171
> [3]:https://gitlab.freedesktop.org/drm/igt-gpu-tools/-/blob/master/tests/kms_plane.c#L1036
> [4]:https://gitlab.freedesktop.org/drm/igt-gpu-tools/-/blob/master/tests/kms_writeback.c
> 
>> Additionally, I noticed the TODO item to add the plane feature (background
>> color KMS property). It seems like a good starting point, and I would be
>> grateful for direction on how to approach it.
> 
> I think this is the good direction, yes!
> 
> All your patches must be on top of drm-misc-next [5]. If you are not
> familiar with kernel contribution, I invite you to look at the kernel
> documentation [6] and at the amazing b4 tool [7].
> 
> For the implementation itself, I think you have to work in the function
> blend, probably around the fill_background function [8]. You also need to
> attach a BACKGROUND property on the CRTC [9]. I think you have to do it
> somewhere in crtc initialization [10].

I implemented this feature to VKMS last year [1], but there were some
discussion about the user-space use of this feature, so the development
got stalled. I encourage you to read through the discussion.

The thing is: when implementing a new DRM uAPI, you need a correspondent
user-space implementation. From the DRM docs [2],

"The short summary is that any addition of DRM uAPI requires
corresponding open-sourced userspace patches, and those patches must be
reviewed and ready for merging into a suitable and canonical upstream
project."

Therefore, it is important to talk to the user-space compositors
community to check their requirements and see if they need this
property.

[1] 
https://lore.kernel.org/dri-devel/20230410125435.128689-1-mcanal@igalia.com/T/
[2] 
https://docs.kernel.org/gpu/drm-uapi.html#open-source-userspace-requirements

Best Regards,
- Maíra

> 
> [5]:https://gitlab.freedesktop.org/drm/misc/kernel/-/tree/drm-misc-next
> [6]:https://docs.kernel.org/process/contribution-maturity-model.html
> [7]:https://b4.docs.kernel.org/
> [8]:https://gitlab.freedesktop.org/drm/misc/kernel/-/blob/drm-misc-next/drivers/gpu/drm/vkms/vkms_composer.c#L389
> [9]:https://drmdb.emersion.fr/properties/3435973836/BACKGROUND
> [10]:https://gitlab.freedesktop.org/drm/misc/kernel/-/blob/drm-misc-next/drivers/gpu/drm/vkms/vkms_crtc.c?ref_type=heads#L273
> 
>> Thank you for your time, and I look forward to your guidance.
>>
>> Best regards,
>> Srikar
>