Re: [PATCH] drm/i915: Correctly populate user mode h/vdisplay with pipe src size during readout

[Jani Nikula <jani.nikula@linux.intel.com>]

On Wed, 02 May 2018, Chris Wilson <chris@chris-wilson.co.uk> wrote:
> Quoting Ville Syrjälä (2018-05-02 17:14:21)
>> On Wed, May 02, 2018 at 04:57:09PM +0100, Chris Wilson wrote:
>> > Quoting Ville Syrjälä (2018-05-02 16:52:41)
>> > > On Wed, May 02, 2018 at 04:33:30PM +0100, Chris Wilson wrote:
>> > > > Quoting Ville Syrjala (2018-04-26 17:30:15)
>> > > > > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
>> > > > > 
>> > > > > During state readout we first read out the pipe src size, store
>> > > > > that information in the user mode h/vdisplay, but later on we overwrite
>> > > > > that with the actual crtc timings. That makes our read out crtc state
>> > > > > inconsistent with itself when the BIOS has enabled the panel fitter to
>> > > > > scale the pipe contents. Let's preserve the pipe src size based
>> > > > > information in the user mode to make things consistent again.
>> > > > 
>> > > > The question I don't feel answered is: If this is the BIOS mode, why
>> > > > aren't we filling it from get_hw_state?
>> > > 
>> > > I suppose the answer is that we're only filling out the bare minimum
>> > > of information during the basic readout. That is everything we need
>> > > for intel_pipe_config_compare() to do its job. Later on we fill the
>> > > gaps to make the state actually presentable to userspace. We don't
>> > > have to do that if the state we read out isn't actually going to be
>> > > exposed to userspace.
>> > > 
>> > > I suppose we could consider doing a more thorough job up front, but
>> > > I think we'd need to spend some though on eg. the handling of the
>> > > mode blob. We probably wouldn't want userspace to gain access to
>> > > our short lived internal mode blob created from the read out state.
>> > 
>> > Will we run into a problem where we say the current mode is 800x600, but
>> > is in fact 1024x768 scaledfrom 800x600? E.g. if we for whatever reason
>> > want to switch to a real 800x600 mode?
>> 
>> Seems unlikely that the real 800x600 mode would have the same blanking
>> lengths and clock as the 1024x768 mode. So we should end up with a full
>> modeset.
>
> Right, that's going to be pretty weird and unlikely.
>
> Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>

From [1],

Tested-by: Larry Finger <Larry.Finger@lwfinger.net>

BR,
Jani.


[1] http://mid.mail-archive.com/4371fd28-49fb-f019-1fc3-f1318b9562fd@lwfinger.net


>
> I guess you would want to throw in a comment as to why this is a special
> case... But this whole pass is pretty special inheritance code...
> -Chris
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-- 
Jani Nikula, Intel Open Source Technology Center
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