[PATCH] drm/ast: Don't check new mode if CRTC is being disabled

[Emil Velikov]

On Mon, 4 May 2020 at 13:07, Thomas Zimmermann <tzimmermann at suse.de> wrote:
>
> Hi Emil
>
> Am 01.05.20 um 15:20 schrieb Emil Velikov:
> > Hi Thomas,
> >
> > Couple of fly-by ideas/suggestions.
> >
> > On Thu, 30 Apr 2020 at 10:13, Thomas Zimmermann <tzimmermann at suse.de> wrote:
> >>
> >> Suspending failed because there's no mode if the CRTC is being
> >> disabled. Early-out in this case. This fixes runtime PM for ast.
> >>
> >> Signed-off-by: Thomas Zimmermann <tzimmermann at suse.de>
> >> ---
> >>  drivers/gpu/drm/ast/ast_mode.c | 3 +++
> >>  1 file changed, 3 insertions(+)
> >>
> >> diff --git a/drivers/gpu/drm/ast/ast_mode.c b/drivers/gpu/drm/ast/ast_mode.c
> >> index 7a9f20a2fd303..089b7d9a0cf3f 100644
> >> --- a/drivers/gpu/drm/ast/ast_mode.c
> >> +++ b/drivers/gpu/drm/ast/ast_mode.c
> >> @@ -801,6 +801,9 @@ static int ast_crtc_helper_atomic_check(struct drm_crtc *crtc,
> >>                 return -EINVAL;
> > Unrelated:
> > This feels quite dirty. If AST1180 does not support atomic modeset
> > simply remove the DRIVER_ATOMIC bit.
> > You can do that at runtime, via drm_device::driver_features in say,
> > ast_detect_chip()?
>
> The line you commented on dates back to non-atomic modesetting, but I
> don't know what the story behind AST1180 is. It is explicitly disabled
> in the list of PCI IDs, but the driver has plenty of code for it. It
> looks as if the chip can only do pageflipping with a pre-set video mode.
>
> As it is right now, the AST1180 code could probably be deleted entirely.
>
No modeset support at all? Ouch.

Removing is one option a shorter/simpler one will be to expose zero connectors.
So any crazy^W brave soul can reinstate AST1180 support.

In either way - it's something for another day/series.

> >
> > The drm_driver::driver_features is immutable, or it ought to be.
> >
> >>         }
> >>
> >> +       if (!state->enable)
> >> +               return 0; /* no checks required if CRTC is being disabled */
> >> +
> > I cannot think of a reason why a driver would need to perform
> > crtc_atomic_check, if the crtc is being disabled.
> > Can you spot any? If not, this should be better served in core, which
> > calls this callback.
> > Correct?
> Ast is a bit of a special case, because it tests the incoming mode
> against a list of re-defined modes. With the crtc being disabled, the
> incoming mode is 0 in all fields. Obviously that's not a valid mode, and
> we need that additional test here.
>
> In the general case, I'd see 'crtc check' as part of the larger atomic
> infrastructure. I can imagine that configurations require the CRTC to be
> enabled before other HW blocks work. So a driver might have a reason to
> run crtc's check even for disabled crtcs (at least to verify that the
> crtc is not disabled). I don't think this can be handled in the core easily.
>
Ack, makes sense.

Thanks
Emil