Re: [PATCH] drm/i915: fix backlight after resume on 855gm

[Chris Wilson <chris@chris-wilson.co.uk>]

On Fri, Jun 26, 2015 at 01:54:18PM +0300, Jani Nikula wrote:
> Some 855gm models (at least ThinkPad X40) regressed because of
> 
> commit b0cd324faed23d10d66ba6ade66579c681feef6f
> Author: Jani Nikula <jani.nikula@intel.com>
> Date:   Wed Nov 12 16:25:43 2014 +0200
> 
>     drm/i915: don't save/restore backlight hist ctl registers
> 
> which tried to make our driver more robust by not blindly saving and
> restoring registers, but it failed to take into account
> 
> commit 0eb96d6ed38430b72897adde58f5477a6b71757a
> Author: Jesse Barnes <jbarnes@virtuousgeek.org>
> Date:   Wed Oct 14 12:33:41 2009 -0700
> 
>     drm/i915: save/restore BLC histogram control reg across suspend/resume
> 
> Fix the regression by enabling hist ctl on gen2.
> 
> v2: Improved the comment.
> 
> Reported-and-tested-by: Philipp Gesang <phg@phi-gamma.net>
> References: http://mid.gmane.org/20150623222648.GD12335@acheron
> Fixes: b0cd324faed2 ("drm/i915: don't save/restore backlight hist ctl registers")
> Cc: Ville Syrj�l� <ville.syrjala@linux.intel.com>
> Cc: stable@vger.kernel.org
> Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
> Signed-off-by: Jani Nikula <jani.nikula@intel.com>
> ---
>  drivers/gpu/drm/i915/i915_reg.h    | 1 +
>  drivers/gpu/drm/i915/intel_panel.c | 7 +++++++
>  2 files changed, 8 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
> index fa9ccb87eb66..bf7c08b94088 100644
> --- a/drivers/gpu/drm/i915/i915_reg.h
> +++ b/drivers/gpu/drm/i915/i915_reg.h
> @@ -3507,6 +3507,7 @@ enum skl_disp_power_wells {
>  #define   BLM_POLARITY_PNV			(1 << 0) /* pnv only */
>  
>  #define BLC_HIST_CTL	(dev_priv->info.display_mmio_offset + 0x61260)
> +#define  BLM_HISTOGRAM_ENABLE			(1 << 31)
>  
>  /* New registers for PCH-split platforms. Safe where new bits show up, the
>   * register layout machtes with gen4 BLC_PWM_CTL[12]. */
> diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
> index 7d83527f95f7..a55477c59559 100644
> --- a/drivers/gpu/drm/i915/intel_panel.c
> +++ b/drivers/gpu/drm/i915/intel_panel.c
> @@ -907,6 +907,13 @@ static void i9xx_enable_backlight(struct intel_connector *connector)
>  
>  	/* XXX: combine this into above write? */
>  	intel_panel_actually_set_backlight(connector, panel->backlight.level);
> +
> +	/*
> +	 * Needed to enable backlight on some 855gm models.

         * BLC_HIST_CTL is 855gm only, but checking for gen2 is

> +	 * safe, as 855gm is the only gen2 that has backlight.
> +	 */

I just felt the second sentence lacked a little context to justify
itself.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre