libweston as a DRM lease lessee

[Damian Hobson-Garcia]

Hi Scott,

On 2021/02/01 19:21, Scott Anderson wrote:
> On 1/02/21 10:19 pm, Damian Hobson-Garcia wrote:
>> Hi all,
>>
>> I am working on adding DRM lease support to a libweston based 
>> compositor.
>> The compositor will be a client (lessee) that will display output to 
>> a DRM lease that
>> is created by another (lessor) process, so this is kind of the 
>> opposite direction to the DRM lease patches
>> that were submitted a while back [1].
>>
>> The motivation is to be able to run multiple instances of weston w/ 
>> DRM backend, where each instance
>> has direct access to a subset of the DRM connectors.  Each instance 
>> could, for example, run in a separate container,
>> with minimal host interaction.
>>
>> In this configuration the DRM lease would be received from a UNIX 
>> domain socket connection to the lessor,
>> so would not discoverable via udev, in the same way that DRM device 
>> nodes normally are.
>>
>> I think that I would need to make changes to the compositor-drm, but 
>> if possible,
>> I'd like to make those changes generic enough to be useful upstream 
>> as well, so I was hoping to get some feedback before possibly heading 
>> down a wrong path.
>>
>>  From what I can tell, I need to:
>>
>> 1. Get the DRM lease file descriptor, given an identifier (In my DRM 
>> lease case this is a name that maps to a socket path)
>> 2. Get a udev_device struct for the device corresponding to the above 
>> fd (via the major:minor numbers)
>>
>> I think that #1 can be implemented in either via the launcher API (a 
>> new launcher type?) or by adding an option for
>> the compositor to provide the fd, but #2 seems like it should be in 
>> compositor-drm, right?
>>
>> Are there other use cases that would benefit from extending the 
>> launcher / compostior APIs in this way?
>> or the udev device handle creation? Are there any plans/interest for 
>> running the DRM backend from inside
>> a container?
>>
>> Thank you,
>> Damian
>>
>> [1] 
>> https://lists.freedesktop.org/archives/wayland-devel/2018-February/037162.html 
>
>
> This doesn't answer any of your questions about Weston specifically, 
> but an alternative approach would be to add drm-lease support to 
> logind. It already has its own concept of groups of devices (seats), 
> so it would also solve any issues regarding who gets what input 
> devices, and the solution wouldn't be specific to weston. Any 
> logind-capable display server (i.e. all of them, including Xorg) would 
> be able to use this, without requiring any modification.
>
> Scott

Thank you for your suggestion.  I'm not too familiar with all of the 
specifics of logind, but I will look into it.  One thing that I noticed
though is that it looks like launcher-logind requests file descriptors 
based on major and minor device numbers.  Since all DRM leases
for a device share the same major/minor, it looks like the TakeDevice() 
method might not be able to differentiate them.  I'll look into it some 
more.

Thank you,
Damian