Re: [PATCH 00/12] misc/syncobj: add /dev/syncobj device

["Michel Dänzer" <michel.daenzer@mailbox.org>]

On 5/19/26 18:00, Christian König wrote:
> On 5/19/26 17:31, Xaver Hugl wrote:
>> Am Di., 19. Mai 2026 um 15:29 Uhr schrieb Christian König
>> <christian.koenig@amd.com>:
>>>> 1. This series makes the ability to manipulate syncobjs available
>>>> independently of attached hardware.
>>>> 2. It makes it available under a consistent path /dev/syncobj.
>>>
>>> Exactly that is a big no-go. This has to be under /dev/dri.
>> FWIW udmabuf is also under /dev directly, but I don't think any
>> compositor developer would complain about a different path.
>> What are the rules for that? Could this simply be put in /dev/dri/syncobj?
> 
> The syncobj are actually the DRM specific way of doing things. The general kernel wide way is to use sync files (see drivers/dma-buf/sync_file.c).
> 
> But there has already been tons of problems with those sync files. E.g. they doesn't support your use case at all since they don't have wait before submit behavior.
> 
> So there are already ways to do this, but the Linux kernel so far told everybody that this is forbidden. The DRM syncobj wait before signal functionality is much better, but then basically the second try to do this.

I'm not quite sure what you're getting at here, just to be clear though:

While the syncobj Wayland protocol extension supports wait-before-submit behaviour at the Wayland protocol level, it doesn't need or cause wait-before-submit behaviour for DMA fences in the kernel. The usual rules apply to fences attached to syncobj timeline points. The wait-before-submit behaviour at the Wayland protocol level comes from allowing submit before a fence is attached to the acquire timeline point.

(It took me a while to realize this distinction, before which I mistakenly thought the kernel's DMA fence rules would prohibit wait-before-submit behaviour at the Wayland protocol level as well)


-- 
Earthling Michel Dänzer       \        GNOME / Xwayland / Mesa developer
https://redhat.com             \               Libre software enthusiast