Re: [PATCH] i915: Introduce quirk for shifting eDP brightness.

From: Puthikorn Voravootivat
Date: Thu Sep 17 2020 - 14:16:28 EST

The Lyude fde7266fb2f6 change is actually based on Chromium change
(https://crrev.com/c/1650325) that fixes the brightness for Samsung
Galaxy Chromebook. So currently we have 2 examples that use LSB
interpretation and 1 that use MSB.


On Thu, Sep 17, 2020 at 10:55 AM Kevin Chowski <chowski@xxxxxxxxxxxx> wrote:
>
> Apologies for being too vague. To be as precise I can be, here is the
> specific code delta I tested: https://crrev.com/c/2406616 . To answer
> your other question, the code I tested against is indeed including the
> fde7266fb2f6 (despite ostensibly being called 5.4 in my commit
> message): our current top-of-tree for our 5.4 branch includes the
> intel_dp_aux_calc_max_backlight logic. Further, I'll note that change
> is exactly the change which breaks my Pixelbook model: prior to the
> change, the max_brightness was hard-coded to 0xFFFF and the math
> worked out that it didn't matter that the hardware cared about the MSB
> despite the driver code caring about the LSB.
>
> To answer Ville's question: the fde7266fb2f6 change which fixes one
> laptop (I believe Thinkpad X1 extreme Gen 2, from some bug reports I
> dug up) and breaks another (Pixelbook); so unfortunately I believe we
> need a quirk at least for some laptop. Reading through the copy of the
> datasheet I have, it wasn't clear to me which was the correct
> interpretation. I'm cc'ing puthik@, who was leaning toward the current
> kernel code (caring about LSB) being the correct interpretation. I
> believe we have other chromebooks which do rely on LSB functionality,
> so unless we can find more examples of laptops wanting MSB it
> currently looks like Pixelbook is the outlier.
>
> On Thu, Sep 17, 2020 at 11:28 AM Jani Nikula
> <jani.nikula@xxxxxxxxxxxxxxx> wrote:
> >
> > On Thu, 17 Sep 2020, Kevin Chowski <chowski@xxxxxxxxxxxx> wrote:
> > > We have observed that Google Pixelbook's backlight hardware is
> > > interpretting these backlight bits from the most-significant side of the
> > > 16 bit word (if DP_EDP_PWMGEN_BIT_COUNT < 16), whereas the driver code
> > > assumes the peripheral cares about the least-significant bits.
> > >
> > > Testing was done from within Chrome OS's build environment when the
> > > patch is backported to 5.4 (the version we are newly targeting for the
> > > Pixelbook); for the record:
> > > $ emerge-eve-kernelnext sys-kernel/chromeos-kernel-5_4 && \
> > > ./update_kernel.sh --remote=$IP
> > >
> > > I used `/sys/kernel/debug/dri/0/eDP-1/i915_dpcd` on my laptop to verify
> > > that the registers were being set according to what the actual hardware
> > > expects; I also observe that the backlight is noticeably brighter with
> > > this patch.
> >
> > It's unclear to me what kernel version this is against, and what you've
> > actually tested.
> >
> > Have you tried v5.7 kernel with Lyude's fde7266fb2f6 ("drm/i915: Fix eDP
> > DPCD aux max backlight calculations")?
> >
> > I just want to make sure you've tested with all the relevant fixes
> > before adding quirks.
> >
> > BR,
> > Jani.
> >
> > >
> > > Signed-off-by: Kevin Chowski <chowski@xxxxxxxxxxxx>
> > > ---
> > >
> > > .../drm/i915/display/intel_dp_aux_backlight.c | 34 +++++++++++++++++++
> > > drivers/gpu/drm/i915/display/intel_quirks.c | 13 +++++++
> > > drivers/gpu/drm/i915/i915_drv.h | 1 +
> > > 3 files changed, 48 insertions(+)
> > >
> > > diff --git a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
> > > index acbd7eb66cbe3..99c98f217356d 100644
> > > --- a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
> > > +++ b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
> > > @@ -91,6 +91,23 @@ static u32 intel_dp_aux_get_backlight(struct intel_connector *connector)
> > > if (intel_dp->edp_dpcd[2] & DP_EDP_BACKLIGHT_BRIGHTNESS_BYTE_COUNT)
> > > level = (read_val[0] << 8 | read_val[1]);
> > >
> > > + if (i915->quirks & QUIRK_SHIFT_EDP_BACKLIGHT_BRIGHTNESS) {
> > > + if (!drm_dp_dpcd_readb(&intel_dp->aux, DP_EDP_PWMGEN_BIT_COUNT,
> > > + &read_val[0])) {
> > > + DRM_DEBUG_KMS("Failed to read DPCD register 0x%x\n",
> > > + DP_EDP_PWMGEN_BIT_COUNT);
> > > + return 0;
> > > + }
> > > + // Only bits 4:0 are used, 7:5 are reserved.
> > > + read_val[0] = read_val[0] & 0x1F;
> > > + if (read_val[0] > 16) {
> > > + DRM_DEBUG_KMS("Invalid DP_EDP_PWNGEN_BIT_COUNT 0x%X, expected at most 16\n",
> > > + read_val[0]);
> > > + return 0;
> > > + }
> > > + level >>= 16 - read_val[0];
> > > + }
> > > +
> > > return level;
> > > }
> > >
> > > @@ -106,6 +123,23 @@ intel_dp_aux_set_backlight(const struct drm_connector_state *conn_state, u32 lev
> > > struct drm_i915_private *i915 = dp_to_i915(intel_dp);
> > > u8 vals[2] = { 0x0 };
> > >
> > > + if (i915->quirks & QUIRK_SHIFT_EDP_BACKLIGHT_BRIGHTNESS) {
> > > + if (!drm_dp_dpcd_readb(&intel_dp->aux, DP_EDP_PWMGEN_BIT_COUNT,
> > > + &vals[0])) {
> > > + DRM_DEBUG_KMS("Failed to write aux backlight level: Failed to read DPCD register 0x%x\n",
> > > + DP_EDP_PWMGEN_BIT_COUNT);
> > > + return;
> > > + }
> > > + // Only bits 4:0 are used, 7:5 are reserved.
> > > + vals[0] = vals[0] & 0x1F;
> > > + if (vals[0] > 16) {
> > > + DRM_DEBUG_KMS("Failed to write aux backlight level: Invalid DP_EDP_PWNGEN_BIT_COUNT 0x%X, expected at most 16\n",
> > > + vals[0]);
> > > + return;
> > > + }
> > > + level <<= (16 - vals[0]) & 0xFFFF;
> > > + }
> > > +
> > > vals[0] = level;
> > >
> > > /* Write the MSB and/or LSB */
> > > diff --git a/drivers/gpu/drm/i915/display/intel_quirks.c b/drivers/gpu/drm/i915/display/intel_quirks.c
> > > index 46beb155d835f..63b27d49b2864 100644
> > > --- a/drivers/gpu/drm/i915/display/intel_quirks.c
> > > +++ b/drivers/gpu/drm/i915/display/intel_quirks.c
> > > @@ -53,6 +53,16 @@ static void quirk_increase_ddi_disabled_time(struct drm_i915_private *i915)
> > > drm_info(&i915->drm, "Applying Increase DDI Disabled quirk\n");
> > > }
> > >
> > > +/*
> > > + * Some eDP backlight hardware uses the most-significant bits of the brightness
> > > + * register, so brightness values must be shifted first.
> > > + */
> > > +static void quirk_shift_edp_backlight_brightness(struct drm_i915_private *i915)
> > > +{
> > > + i915->quirks |= QUIRK_SHIFT_EDP_BACKLIGHT_BRIGHTNESS;
> > > + DRM_INFO("Applying shift eDP backlight brightness quirk\n");
> > > +}
> > > +
> > > struct intel_quirk {
> > > int device;
> > > int subsystem_vendor;
> > > @@ -156,6 +166,9 @@ static struct intel_quirk intel_quirks[] = {
> > > /* ASRock ITX*/
> > > { 0x3185, 0x1849, 0x2212, quirk_increase_ddi_disabled_time },
> > > { 0x3184, 0x1849, 0x2212, quirk_increase_ddi_disabled_time },
> > > +
> > > + /* Google Pixelbook */
> > > + { 0x591E, 0x8086, 0x2212, quirk_shift_edp_backlight_brightness },
> > > };
> > >
> > > void intel_init_quirks(struct drm_i915_private *i915)
> > > diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> > > index e4f7f6518945b..cc93bede4fab8 100644
> > > --- a/drivers/gpu/drm/i915/i915_drv.h
> > > +++ b/drivers/gpu/drm/i915/i915_drv.h
> > > @@ -525,6 +525,7 @@ struct i915_psr {
> > > #define QUIRK_PIN_SWIZZLED_PAGES (1<<5)
> > > #define QUIRK_INCREASE_T12_DELAY (1<<6)
> > > #define QUIRK_INCREASE_DDI_DISABLED_TIME (1<<7)
> > > +#define QUIRK_SHIFT_EDP_BACKLIGHT_BRIGHTNESS (1<<8)
> > >
> > > struct intel_fbdev;
> > > struct intel_fbc_work;
> >
> > --
> > Jani Nikula, Intel Open Source Graphics Center