Re: Broken LVDS output at mode changes

[Daniel Vetter <daniel@ffwll.ch>]

On Wed, Mar 28, 2012 at 03:29:04PM +0200, Takashi Iwai wrote:
> Hi,
> 
> we've encountered a broken LVDS output on some IVY/SNB machines when
> the mode is changed (from/to native resolution).  When this happens,
> the whole laptop panel gets half white and half black.  This doesn't
> recover until the LVDS is turned off once.
> 
> And, there is no signficant difference between working and non-working
> cases in the register dumps.  From the software POV, all looks sane.
> So, we suspect this is rather specific to some panel hardware.
> 
> However, through debugging, I found that disabling LVDS at mode change
> works around the problem.  A test patch is attached below.
> 
> My question now is: can this workaround have any serious drawback?
> I thought of a longer blank time, but I didn't notice any difference
> before and after the patch.
> 
> Or, any other suggestion as a saner fix?

No idea, I'm wondering though whether we should just accept some
flickering while modesetting unconditionally. Does anyone know what
Windows does in this case and at least on my work machine here it looks
like Windows just blanks the screen. I haven't checked with reg dumps
though how exactly they upscale stuff on lvds.

git blame says that Chris Wilson created the original PCH_SPLIT check.
Chris, any comments on this?

Yours, Daniel
> 
> 
> thanks,
> 
> Takashi
> 
> ---
> ---
>  drivers/gpu/drm/i915/intel_lvds.c |   23 ++++++++++++++++++++++-
>  1 file changed, 22 insertions(+), 1 deletion(-)
> 
> --- a/drivers/gpu/drm/i915/intel_lvds.c
> +++ b/drivers/gpu/drm/i915/intel_lvds.c
> @@ -398,6 +398,26 @@ out:
>  	return true;
>  }
>  
> +static int intel_disable_at_modeset_callback(const struct dmi_system_id *id)
> +{
> +	DRM_DEBUG_KMS("Disabling LVDS at modeset for %s\n", id->ident);
> +	return 1;
> +}
> +
> +/* Requires to disable LVDS for avoiding a broken screen at mode change */
> +static const struct dmi_system_id intel_disable_at_modeset[] = {
> +	{
> +		.callback = intel_disable_at_modeset_callback,
> +		.ident = "HP ProBook",
> +		.matches = {
> +			DMI_MATCH(DMI_SYS_VENDOR, "Hewlett-Packard"),
> +			DMI_MATCH(DMI_PRODUCT_NAME, "HP ProBook"),
> +		},
> +	},
> +
> +	{ }	/* terminating entry */
> +};
> +
>  static void intel_lvds_prepare(struct drm_encoder *encoder)
>  {
>  	struct intel_lvds *intel_lvds = to_intel_lvds(encoder);
> @@ -407,7 +427,8 @@ static void intel_lvds_prepare(struct dr
>  	 * the panel fitter. However at all other times we can just reset
>  	 * the registers regardless.
>  	 */
> -	if (!HAS_PCH_SPLIT(encoder->dev) && intel_lvds->pfit_dirty)
> +	if ((!HAS_PCH_SPLIT(encoder->dev) && intel_lvds->pfit_dirty) ||
> +	    dmi_check_system(intel_disable_at_modeset))
>  		intel_lvds_disable(intel_lvds);
>  }
>  
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-- 
Daniel Vetter
Mail: daniel@ffwll.ch
Mobile: +41 (0)79 365 57 48