Re: [PATCH 2/2] drm/i915/dp: workaround BIOS eDP bpp clamping issue

[Jani Nikula <jani.nikula@intel.com>]

On Wed, 16 Oct 2013, Jani Nikula <jani.nikula@intel.com> wrote:
> This isn't a real fix to the problem, but rather a stopgap measure while
> trying to find a proper solution.
>
> There are several laptops out there that fail to light up the eDP panel
> in UEFI boot mode. They seem to be mostly IVB machines, including but
> apparently not limited to Dell XPS 13, Asus TX300, Asus UX31A, Asus
> UX32VD, Acer Aspire S7. They seem to work in CSM or legacy boot.
>
> The difference between UEFI and CSM is that the BIOS provides a
> different VBT to the kernel. The UEFI VBT typically specifies 18 bpp and
> 1.62 GHz link for eDP, while CSM VBT has 24 bpp and 2.7 GHz link. We end
> up clamping to 18 bpp in UEFI mode, which we can fit in the 1.62 Ghz
> link, and for reasons yet unknown fail to light up the panel.
>
> Dithering from 24 to 18 bpp itself seems to work; if we use 18 bpp with
> 2.7 GHz link, the eDP panel lights up. So essentially this is a link
> speed issue, and *not* a bpp clamping issue.
>
> The bug raised its head since
> commit 657445fe8660100ad174600ebfa61536392b7624
> Author: Daniel Vetter <daniel.vetter@ffwll.ch>
> Date:   Sat May 4 10:09:18 2013 +0200
>
>     Revert "drm/i915: revert eDP bpp clamping code changes"
>
> which started clamping bpp *before* computing the link requirements, and
> thus affecting the required bandwidth. Clamping after the computations
> kept the link at 2.7 GHz.
>
> Even though the BIOS tells us to use 18 bpp through the VBT, it happily
> boots up at 24 bpp and 2.7 GHz itself! Use this information to
> selectively ignore the VBT provided value.
>
> We can't ignore the VBT eDP bpp altogether, as there are other laptops
> that do require the clamping to be used due to EDID reporting higher bpp
> than the panel can support.
>
> Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=59841
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=67950
> CC: stable@vger.kernel.org
> Signed-off-by: Jani Nikula <jani.nikula@intel.com>

Tested-by: Ulf Winkelvos <ulf@winkelvos.de>
Tested-by: jkp <jkp@iki.fi>

and I also have another machine on loan this fixes.

Jani.


>
> ---
>
> For backporting to v3.12, you also need
> commit 84a44adc16ab118cf7e0518861216cbc91cee6e4
> Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Date:   Fri Sep 6 23:29:00 2013 +0300
>
>     drm/i915: Add support for pipe_bpp readout
> ---
>  drivers/gpu/drm/i915/intel_dp.c |   21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> index f63aa8c..cb895d8 100644
> --- a/drivers/gpu/drm/i915/intel_dp.c
> +++ b/drivers/gpu/drm/i915/intel_dp.c
> @@ -1476,6 +1476,27 @@ static void intel_dp_get_config(struct intel_encoder *encoder,
>  		ironlake_check_encoder_dotclock(pipe_config, dotclock);
>  
>  	pipe_config->adjusted_mode.crtc_clock = dotclock;
> +
> +	if (is_edp(intel_dp) && dev_priv->vbt.edp_bpp &&
> +	    pipe_config->pipe_bpp > dev_priv->vbt.edp_bpp) {
> +		/*
> +		 * This is a big fat ugly hack.
> +		 *
> +		 * Some machines in UEFI boot mode provide us a VBT that has 18
> +		 * bpp and 1.62 GHz link bandwidth for eDP, which for reasons
> +		 * unknown we fail to light up. Yet the same BIOS boots up with
> +		 * 24 bpp and 2.7 GHz link. Use the same bpp as the BIOS uses as
> +		 * max, not what it tells us to use.
> +		 *
> +		 * Note: This will still be broken if the eDP panel is not lit
> +		 * up by the BIOS, and thus we can't get the mode at module
> +		 * load.
> +		 */
> +		DRM_DEBUG_KMS("pipe has %d bpp for eDP panel, overriding BIOS-provided max %d bpp\n",
> +			      pipe_config->pipe_bpp, dev_priv->vbt.edp_bpp);
> +		dev_priv->vbt.edp_bpp = pipe_config->pipe_bpp;
> +	}
> +
>  }
>  
>  static bool is_edp_psr(struct drm_device *dev)
> -- 
> 1.7.9.5
>

-- 
Jani Nikula, Intel Open Source Technology Center
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