Re: [PATCH] drm/i915: Don't write DSPSURF for old chips

[Takashi Iwai <tiwai@suse.de>]

At Mon, 27 Feb 2012 12:17:57 +0000,
Chris Wilson wrote:
> 
> On Mon, 27 Feb 2012 12:39:24 +0100, Takashi Iwai <tiwai@suse.de> wrote:
> > It seems that writing DSPSURF in intel_flush_display_plane() causes
> > the blank screen on some old laptops like Dell D630 with 965GM.
> > Since this operation is needed only for ILK+, make it conditional.
> 
> The specs say that DSPASURF is the latch register for updates of the DSPA
> registers on gen4 (including 965gm) as well. Presumably the bug is that
> we only partially update the DSPA registers prior to the first call to
> intel_flush_display_plane() which this papers over by disabling the
> update until a valid address is written to DSPASURF. And there is such a
> spurious call to intel_enable_plane() prior to us setting a valid
> scanout surface:

Sounds reasonable.  FWIW, the change was first introduced in commit
[b24e7179: drm/i915: add pipe/plane enable/disable functions],
then in commit [efc2924e: drm/i915: Call intel_enable_plane from
i9xx_crtc_mode_set (again)], it's placed into i9xx_crtc_mode_set().

This explains the fact that it was discovered only on old machines
as i9xx_crtc_mode_set() is the only crtc_mode_set op calling
intel_enable_plane().

BTW, the bisection leaded to a merge commit, so the bug is really
depending on the activation path or timing.

I'll ask a tester to try your patch.


thanks,

Takashi

> diff --git a/drivers/gpu/drm/i915/intel_display.c
> b/drivers/gpu/drm/i915/intel_d
> index 66b19d3..ea8e4a1 100644
> --- a/drivers/gpu/drm/i915/intel_display.c
> +++ b/drivers/gpu/drm/i915/intel_display.c
> @@ -5453,7 +5453,6 @@ static int i9xx_crtc_mode_set(struct drm_crtc
> *crtc,
>  
>         I915_WRITE(DSPCNTR(plane), dspcntr);
>         POSTING_READ(DSPCNTR(plane));
> -       intel_enable_plane(dev_priv, plane, pipe);
>  
>         ret = intel_pipe_set_base(crtc, x, y, old_fb);
> 
> -Chris
> 
> -- 
> Chris Wilson, Intel Open Source Technology Centre
>