[Intel-gfx] [PATCH 1/6] drm: Let userspace check if driver supports modeset

[Rodrigo Vivi]

On Tue, Jul 17, 2018 at 09:04:45PM +0100, Chris Wilson wrote:
> Quoting Souza, Jose (2018-07-17 18:02:17)
> > On Tue, 2018-07-17 at 08:28 +0100, Chris Wilson wrote:
> > > Quoting José Roberto de Souza (2018-07-16 23:38:36)
> > > > GPU accelerators usually don't have display block or the display
> > > > driver part can be disable when building driver(for servers it save
> > > > some resources) so it is important let userspace check this
> > > > capability too.
> > > 
> > > We currently communicate that by having no modeset resources. What
> > > does
> > > another cap bit accomplish that we don't already know?
> > 
> > This is a hackish way to check if driver support modeset,
> > drmModeGetResources()/drm_mode_getresources() can fail and return null
> > by other reasons and just check for the errno value can be misleading
> > too.
> 
> Do not confuse libdrm with the ioctl. You do not need to allocate
> anything for such a check, and it is being used directly without
> allocations as a has-kms check.
> 
> More to the point existing userspace determines modeset capability
> through the reported resources. Your changelog needs to explain why they
> are inadequate and how you plan to coordinate your fix with userspace.
> As it stands you are introducing uABI with no user...

I agree with Chris here. There is no indication on how this will
affect userspace here and this so far is just a uABI without user
that is a big blocker.

And I also got confused because that modeset capability check
got introduced in a block that is for
"/* Only some caps make sense with UMS/render-only drivers. */"

> -Chris
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