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

From: Lyude Paul
Date: Thu Sep 17 2020 - 14:43:47 EST

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On Thu, 2020-09-17 at 21:25 +0300, Ville Syrjälä wrote:
> On Thu, Sep 17, 2020 at 11:14:43AM -0700, Puthikorn Voravootivat wrote:
> > 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.
>
> "If field 4:0 of the EDP_PWMGEN_BIT_COUNT register represents a value
> of greater than 8 and the BACKLIGHT_BRIGHTNESS_BYTE_COUNT bit
> is cleared to 0, only the 8 MSB of the brightness control value can be
> controlled.
> (See Note below.)
> Assigned bits are allocated to the MSB of the enabled register
> combination."
>
> I think that's pretty clear the assigned bits are supposed to be
> msb aligned.

Are we sure that isn't just referring to the DP_EDP_BACKLIGHT_BRIGHTNESS_MSB
register, as opposed to alignment of the whole value in that register? My
understanding of this was just that if there wasn't a pwmgen bit count
specified, that the backlight level would just be written to
DP_EDP_BACKLIGHT_BRIGHTNESS_MSB and DP_EDP_BACKLIGHT_BRIGHTNESS_LSB would be
ignored.

Hopefully I'm not misunderstanding Ville, but I don't think so since the current
code we have already follows the understanding I just gave:

vals[0] = level;

/* Write the MSB and/or LSB */
if (intel_dp->edp_dpcd[2] & DP_EDP_BACKLIGHT_BRIGHTNESS_BYTE_COUNT) {
vals[0] = (level & 0xFF00) >> 8;
vals[1] = (level & 0xFF);
}

(vals[0] == 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
--
Sincerely,
Lyude Paul (she/her)
Software Engineer at Red Hat

Next message: Krzysztof Kozlowski: "[PATCH v2 00/13] gpio: add common dtschema"
Previous message: Jonathan Cameron: "Re: [PATCH 3/3] iio: dac: ad5592r: localize locks only where needed in ad5592r_read_raw()"
In reply to: Puthikorn Voravootivat: "Re: [PATCH] i915: Introduce quirk for shifting eDP brightness."
Next in thread: Ville Syrjälä: "Re: [Intel-gfx] [PATCH] i915: Introduce quirk for shifting eDP brightness."
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