[Intel-gfx] [PATCH 3/3] drm/i915: Add code comment on assumption of pipe==transcoder

[Imre Deak]

On Mon, Nov 19, 2018 at 02:06:31PM -0800, Lucas De Marchi wrote:
> On Mon, Nov 19, 2018 at 08:43:27PM +0200, Imre Deak wrote:
> > On Mon, Nov 19, 2018 at 05:51:31PM +0200, Ville Syrjälä wrote:
> > > On Mon, Nov 19, 2018 at 04:41:09PM +0200, Imre Deak wrote:
> > > > Add a comment to the pipe and transcoder enum definitions about our
> > > > assumption in the code that pipe==transcoder for PIPE_A-C /
> > > > TRANSCODER_A-C. This means we have to keep the values for these
> > > > pipe/transcoder enums fixed.
> > > > 
> > > > Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > > > Cc: Lucas De Marchi <lucas.demarchi at intel.com>
> > > > Cc: Mika Kahola <mika.kahola at intel.com>
> > > > Signed-off-by: Imre Deak <imre.deak at intel.com>
> > > > ---
> > > >  drivers/gpu/drm/i915/intel_display.h | 8 ++++++++
> > > >  1 file changed, 8 insertions(+)
> > > > 
> > > > diff --git a/drivers/gpu/drm/i915/intel_display.h b/drivers/gpu/drm/i915/intel_display.h
> > > > index 43eb4ebbcc35..cbb5d79d6a4c 100644
> > > > --- a/drivers/gpu/drm/i915/intel_display.h
> > > > +++ b/drivers/gpu/drm/i915/intel_display.h
> > > > @@ -43,6 +43,10 @@ enum i915_gpio {
> > > >  	GPIOM,
> > > >  };
> > > >  
> > > > +/*
> > > > + * Keep the PIPE_A-C values fixed, we assume that pipe==transcoder for
> > > > + * these pipes.
> > > > + */
> > > 
> > > I suspect the A-C part is going to bitrot. Also I'm sure we
> > > make more assupmtions about these values (PIPE_A == 0,
> > > PIPE_N+1 == n+1, etc.). We should probably try to spell it
> > > all out here.
> > 
> > Ok, can add instead:
> > 
> > /*
> >  * Keep the pipe enum values fixed: the code assumes that PIPE_A=0, the
> >  * rest have consecutive values and match the enum values of transcoders
> >  * with a 1:1 transcoder->pipe mapping.
> 
> 1:1 transcoder <-> pipe mapping, so it doesn't look like it's a pointer :)

You can only map from transcoder to pipe not the other way around (a pipe
can always be connected to any transcoder). But will add spaces around
'->'.

> 
> >  */
> > 
> > > 
> > > >  enum pipe {
> > > >  	INVALID_PIPE = -1,
> > > >  
> > > > @@ -56,6 +60,10 @@ enum pipe {
> > > >  
> > > >  #define pipe_name(p) ((p) + 'A')
> > > >  
> > > > +/*
> > > > + * Keep the TRANSCODER_A-C values fixed, we assume that pipe==transcoder for
> > > > + * these transcoders.
> > > > + */
> > > 
> > > Same issue with the A-C part perhaps.
> > 
> > and here:
> > 
> > > 
> > > >  enum transcoder {
> > 
> > /*
> >  * The following transcoders have a 1:1 transcoder->pipe mapping, keep
> >  * their values fixed: the code assumes that TRANSCODER_A=0, the rest
> >  * have consecutive values and match the enum values of the pipes they map
> >  * to.
> >  */
> > 
> > 
> > > >  	TRANSCODER_A = 0,
> > > >  	TRANSCODER_B,
> > > >     TRANSCODER_C,
> 
> could we additionally do like below?
> 
> TRANSCODER_A = PIPE_A,
> TRANSCODER_B = PIPE_B,
> TRANSCODER_C = PIPE_C,

Good idea, will change it.

> Or at least a BUILD_BUG_ON(TRANSCODER_A != PIPE_A || TRANSCODER_B != PIPE_B || TRANSCODER_C != PIPE_C)

This could get stale.

> 
> Lucas De Marchi
> 
> > 
> > /*
> >  * The following transcoders can map to any pipe, their enum value
> >  * doesn't need to stay fixed.
> >  */
> > 
> > > >     TRANSCODER_EDP,
> > > >     TRANSCODER_DSI_0,
> > > >     TRANSCODER_DSI_1,
> > 
> > 
> > > > -- 
> > > > 2.13.2
> > > 
> > > -- 
> > > Ville Syrjälä
> > > Intel
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