[Intel-gfx] [PATCH v2] drm/i915/edp/jsl: Update vswing table for HBR and HBR2

[Matt Roper]

On Tue, Sep 29, 2020 at 11:30:22PM +0300, Ville Syrjälä wrote:
> On Tue, Sep 29, 2020 at 08:20:22PM +0000, Souza, Jose wrote:
> > On Tue, 2020-09-29 at 23:02 +0300, Ville Syrjälä wrote:
> > > On Tue, Sep 29, 2020 at 07:33:45PM +0000, Souza, Jose wrote:
> > > > On Tue, 2020-09-29 at 17:41 +0530, Tejas Upadhyay wrote:
> > > > > JSL has update in vswing table for eDP
> > > > 
> > > > Would be nice to mention in the commit description why PCH is being used, that would avoid Ville's question.
> > > 
> > > If the thing has nothing to do PCH then it should not use the PCH type
> > > for the the check. Instead we should just do the EHL/JSL split.
> > 
> > In the first version Matt Roper suggested to use PCH to differentiate between EHL and JSL, Jani also agreed with this solution.This 2 PCHs can only be
> > associate with EHL and JSL respectively, so no downsides here.
> 
> The downside is that the code makes no sense on the first glance.
> It's going to generate a "wtf?" exception in the brain and require
> me to take a second look to figure what is going on. Exception
> handling is expensive and shouldn't be needed in cases where it's
> trivial to make the code 100% obvious.

The bspec documents EHL and JSL as being the same platform and identical
in all programming since they are literally the same display IP; this
vswing table is the one and only place where the two are treated in a
distinct manner for reasons that lie outside the display controller.  If
you had to stop and take a closer look at the code here, that's a
probably a good thing since in general there should generally never be a
difference in the behavior between the two.  Adding an additional
clarifying comment is probably in order too since this is a very
exceptional special case.

If we deviate from the bspec's guidance and try to split IS_ELKHARTLAKE
and IS_JASPERLAKE across the whole driver, that's going to be a lot more
pain to maintain down the road since we'll almost certainly have cases
where someone silently leaves one or the other off a condition and gets
unexepcted behavior.  I could see arguments for using a SUBPLATFORM here
like we do for TGL_U vs TGL_Y, but even that seems like overkill if we
already have a clear way to distinguish the two cases (PCH pairing) and
can just leave a clarifying comment.


Matt


> 
> -- 
> Ville Syrjälä
> Intel

-- 
Matt Roper
Graphics Software Engineer
VTT-OSGC Platform Enablement
Intel Corporation
(916) 356-2795