[Intel-gfx] [PATCH] drm/i915: rip out edp special case from dp_link_down

[Paulo Zanoni]

Hi

2012/9/9 Daniel Vetter <daniel.vetter at ffwll.ch>:
> This has been tons of fun to figure out with git blame. The first
> notion of this code block goes back to the original cpu edp enabling
> for ilk in
>
> commit 32f9d658aee5be09ebdd28fc730630e61d0b46db
> Author: Zhenyu Wang <zhenyuw at linux.intel.com>
> Date:   Fri Jul 24 01:00:32 2009 +0800
>
>     drm/i915: Add eDP support on IGDNG mobile chip
>
> Two things are notable in this commit wrt to the this edp special
> case:
> - The IS_eDP check _only_ fires for DP A, i.e. cpu edp ports.
> - The cpu edp port is disabled at the top of the dp_link_down function.
>
> My theory is that these hacks was added to work around the completely
> different modeset sequence for cpu edp ports compared to pch edp
> ports. With the cpu edp confusion on ilk (and snb/ivb) now fixed up,
> this shouldn't be required any more.
>
> The really interesting question is how this special cases survived
> this long in the code. The first step is declaring the pch port D as
> eDP if it's used for an internal panel:
>
> commit b329530ca7cdf6bf014f2124efd983e01265d623
> Author: Adam Jackson <ajax at redhat.com>
> Date:   Fri Jul 16 14:46:28 2010 -0400
>
>     drm/i915/dp: Correctly report eDP in the core connector type
>
> This commit unfortunately failed to notice that not all edp ports are
> created equal. Then follow a flurry of refactorings, culminating in a
> patch from Keith Packard which resulted in the current logic (by
> making it "correct" for all platforms that have edp):
>
> commit 417e822deee1d2bcd8a8a60660c40a0903713f2b
> Author: Keith Packard <keithp at keithp.com>
> Date:   Tue Nov 1 19:54:11 2011 -0700
>
>     drm/i915: Treat PCH eDP like DP in most places
>
> None of these cleanups or refactorings supply any reason why we need
> this code, they've simply carried it on as-is.
>
> Hence presume it might be harmful with the current code and rip it
> out. We do rewrite the link training bits completely anyway when
> re-training the link.
>
> Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>

After looking for a long time I could not find a reason why we should
set the pattern to "train off" while disabling the eDP port. Notice
that "train off" means "send normal pixels" (see BSpec). All the docs
say is that when _enabling_ the port we need to set training pattern
1, which we should already be doing. After your patch, when we disable
the port we will disable it with training pattern already set to 1
(instead of "send normal pixels/ train off"), but there's nothing
saying we shouldn't do this. So yeah, your patch looks fine. If we
ever revert this patch, let's make sure we add a big comment
explaining it.

Reviewed-by: Paulo Zanoni <paulo.r.zanoni at intel.com>

> ---
>  drivers/gpu/drm/i915/intel_dp.c | 7 -------
>  1 file changed, 7 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> index f28353d..8adad5e 100644
> --- a/drivers/gpu/drm/i915/intel_dp.c
> +++ b/drivers/gpu/drm/i915/intel_dp.c
> @@ -1934,13 +1934,6 @@ intel_dp_link_down(struct intel_dp *intel_dp)
>
>         msleep(17);
>
> -       if (is_edp(intel_dp)) {
> -               if (HAS_PCH_CPT(dev) && (IS_GEN7(dev) || !is_cpu_edp(intel_dp)))
> -                       DP |= DP_LINK_TRAIN_OFF_CPT;
> -               else
> -                       DP |= DP_LINK_TRAIN_OFF;
> -       }
> -
>         if (HAS_PCH_IBX(dev) &&
>             I915_READ(intel_dp->output_reg) & DP_PIPEB_SELECT) {
>                 struct drm_crtc *crtc = intel_dp->base.base.crtc;
> --
> 1.7.11.2
>
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-- 
Paulo Zanoni