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

From: Lucas De Marchi <lucas.de.marchi@gmail.com>
To: Imre Deak <imre.deak@intel.com>
Cc: intel-gfx@lists.freedesktop.org,
	Lucas De Marchi <lucas.demarchi@intel.com>
Subject: Re: [PATCH 3/3] drm/i915: Add code comment on assumption of pipe==transcoder
Date: Mon, 19 Nov 2018 14:06:31 -0800	[thread overview]
Message-ID: <20181119220629.GA9928@ldmartin-desk.jf.intel.com> (raw)
In-Reply-To: <20181119184327.GA9568@ideak-desk.fi.intel.com>

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@linux.intel.com>
> > > Cc: Lucas De Marchi <lucas.demarchi@intel.com>
> > > Cc: Mika Kahola <mika.kahola@intel.com>
> > > Signed-off-by: Imre Deak <imre.deak@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 :)

>  */
> 
> > 
> > >  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,

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

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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