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

[Daniel Vetter]

On Mon, May 04, 2020 at 02:07:22PM +0200, Thomas Zimmermann 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.
> 
> > 
> > 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.

Jumping out of ->atomic_check callbacks when stuff is all off is fairly
standard pattern. So much standard that I'm kinda wondering whether we
shouldn't just make it the default in atomic helpers - so many drivers
forget to do it and break in funny ways.
-Daniel

> 
> Best regards
> Thomas
> 
> > 
> > HTH
> > -Emil
> > _______________________________________________
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> > dri-devel at lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/dri-devel
> > 
> 
> -- 
> Thomas Zimmermann
> Graphics Driver Developer
> SUSE Software Solutions Germany GmbH
> Maxfeldstr. 5, 90409 Nürnberg, Germany
> (HRB 36809, AG Nürnberg)
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> 




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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch