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

[Souza, Jose]

On Wed, 2018-08-15 at 14:15 -0700, Rodrigo Vivi wrote:
> 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.

The user would be IGT but I would only send the patch after get this
merged.

Okay, I can drop this patch without any changes in this series.

> 
> 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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