Re: [PATCH v2 13/26] drm/fb_cma_helper: Remove implicit call to disable_unused_functions

[Daniel Vetter]

On Mon, Jan 25, 2016 at 12:19:27AM +0200, Laurent Pinchart wrote:
> Hi Daniel,
> 
> On Friday 15 January 2016 11:17:30 Daniel Vetter wrote:
> > On Fri, Jan 15, 2016 at 01:13:05AM +0200, Laurent Pinchart wrote:
> > > On Thursday 14 January 2016 16:24:56 Maxime Ripard wrote:
> > >> The drm_fbdev_cma_init function always calls the
> > >> drm_helper_disable_unused_functions. Since it's part of the usual probe
> > >> process, all the drivers using that helper will end up having their
> > >> encoder and CRTC disable functions called at probe if their device has
> > >> not been reported as enabled.
> > >> 
> > >> This could be fixed by reading out from the registers the current state
> > >> of the device if it is enabled, but even that will not handle the case
> > >> where the device is actually disabled.
> > >> 
> > >> Moreover, the drivers using the atomic modesetting expect that their
> > >> enable and disable callback to be called when the device is already
> > >> enabled or disabled (respectively).
> > >> 
> > >> We can however fix this issue by moving the call to
> > >> drm_helper_disable_unused_functions out of drm_fbdev_cma_init and make
> > >> the drivers needing it (all the drivers calling drm_fbdev_cma_init and
> > >> not using the atomic modesetting) explicitly call it.
> > > 
> > > I'd rather add it to all drivers that call drm_fbdev_cma_init(). All the
> > > atomic ones must have special code to cope with it that could be rendered
> > > useless by the removal of the drm_helper_disable_unused_functions() call.
> > > That code should be removed too, and it would be easier to check drivers
> > > one by one and fixing them individually (outside of this patch series,
> > > unless you insist ;-)) when removing the explicit
> > > drm_helper_disable_unused_functions() call.
> >
> > I had the same thought, but figured there's really no good reason ever to
> > do this. I suspect most of that disable_unused_function stuff we have all
> > over is supreme cargo-cult anyway ;-)
> 
> I'm not sure you got my point. Yes, the drm_helper_disable_unused_functions() 
> call should be removed from atomic drivers, but that will leave support code 
> added for the sole reason of avoiding the crash in the drivers. That code will 
> likely not be noticed and will stay there rotting. If we added 
> drm_helper_disable_unused_functions() calls to all atomic drivers then driver 
> authors will hopefully check carefully if there's any support code that can be 
> removed when removing the function call. It would act as a kind of FIXME 
> reminder.

drm_helper_disable_unused_functions() already prefers the ->disable
callbacks over dpms, like atomic helpers. I don't think there's any dead
code due to this change. The problem was that driver/hw blew up since it
was disabled when the hw was off already.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch