[PATCH] drm/i915: Program LUT before intel_color_commit() if LUT was not previously set

Hans de Goede hdegoede at redhat.com
Mon Oct 28 18:56:11 UTC 2019


On 25-10-2019 21:45, Ville Syrjälä wrote:
> On Fri, Oct 25, 2019 at 09:23:47PM +0200, Hans de Goede wrote:
>> Hi,
>> On 21-10-2019 16:39, Ville Syrjälä wrote:
>>> On Sun, Oct 20, 2019 at 08:19:33PM +0200, Hans de Goede wrote:
>>>> Since commit 051a6d8d3ca0 ("drm/i915: Move LUT programming to happen after
>>>> vblank waits"), I am seeing an ugly colored flash of the first few display
>>>> lines on 2 Cherry Trail devices when the gamma table gets set for the first
>>>> time. A blue flash on a GPD win and a yellow flash on an Asus T100HA.
>>>> The problem is that since this change, the LUT is programmed after the
>>>> write *and latching* of the double-buffered register which causes the LUT
>>>> to be used starting at the next frame. This means that the old LUT is still
>>>> used for the first couple of lines of the display. If no LUT was in use
>>>> before then the LUT registers may contain bogus values. This leads to
>>>> messed up colors until the new LUT values are written. At least on CHT DSI
>>>> panels this causes messed up colors on the first few lines.
>>>> This commit fixes this by adding a load_lut_before_commit boolean,
>>>> modifying intel_begin_crtc_commit to load the luts earlier if this is set,
>>>> and setting this from intel_color_check when a LUT table was not in use
>>>> before (and thus may contain bogus values), or when the table size
>>>> changes.
>>> The real solution is vblank workers, which I have somewhat implemented
>>> here:
>>> git://github.com/vsyrjala/linux.git vblank_worker_8_kthread
>>> Though even with the qos tricks there we still probably can't quite make
>>> it in time. Essentially we have a bit less than one scanline after start
>>> of vblank to do the work before pixels start to flow through the pipe.
>>> We might be extend that to almost four scanlines but that partocular
>>> thing is documeted as debug feature so not sure we should really use it.
>>> Also I don't think four scanlines is always enough either. So it's still
>>> very much possible that we get the first 100 or so pixels with the old LUT.
>> Thank you for the info and for the review.
>>>> Fixes: 051a6d8d3ca0 ("drm/i915: Move LUT programming to happen after vblank waits")
>>>> Signed-off-by: Hans de Goede <hdegoede at redhat.com>
>>>> ---
>>>>    drivers/gpu/drm/i915/display/intel_color.c    | 26 +++++++++++++++++++
>>>>    drivers/gpu/drm/i915/display/intel_display.c  |  7 +++++
>>>>    .../drm/i915/display/intel_display_types.h    |  3 +++
>>>>    3 files changed, 36 insertions(+)
>>>> diff --git a/drivers/gpu/drm/i915/display/intel_color.c b/drivers/gpu/drm/i915/display/intel_color.c
>>>> index 71a0201437a9..0da6dcc5bebd 100644
>>>> --- a/drivers/gpu/drm/i915/display/intel_color.c
>>>> +++ b/drivers/gpu/drm/i915/display/intel_color.c
>>>> @@ -1052,6 +1052,32 @@ intel_color_add_affected_planes(struct intel_crtc_state *new_crtc_state)
>>>>    		new_crtc_state->update_planes |= BIT(plane->id);
>>>>    	}
>>>> +	/*
>>>> +	 * Normally we load the LUTs after vblank / after the double-buffer
>>>> +	 * registers written by commit have been latched, this avoids a
>>>> +	 * gamma change mid-way the screen. This does mean that the first
>>>> +	 * few lines of the display will (sometimes) still use the old
>>>> +	 * table. This is fine when changing an existing LUT, but if this
>>>> +	 * is the first time the LUT gets loaded, then the hw may contain
>>>> +	 * random values, causing the first lines to have funky colors.
>>>> +	 *
>>>> +	 * So if were enabling a LUT for the first time or changing the table
>>>> +	 * size, then we must do this before the commit to avoid corrupting
>>>> +	 * the first lines of the display.
>>>> +	 */
>>>> +	if (!old_crtc_state->base.gamma_lut && new_crtc_state->base.gamma_lut)
>>>> +		new_crtc_state->load_lut_before_commit = true;
>>>> +	else if (!old_crtc_state->base.degamma_lut &&
>>>> +		 new_crtc_state->base.degamma_lut)
>>>> +		new_crtc_state->load_lut_before_commit = true;
>>>> +	else if (old_crtc_state->base.gamma_lut &&
>>>> +		 new_crtc_state->base.gamma_lut &&
>>>> +		 lut_is_legacy(old_crtc_state->base.gamma_lut) !=
>>>> +			lut_is_legacy(new_crtc_state->base.gamma_lut))
>>>> +		new_crtc_state->load_lut_before_commit = true;
>>>> +	else
>>>> +		new_crtc_state->load_lut_before_commit = false;
>>> The 'no gamma -> yes gamma' thing I might be willing to accept. The rest
>>> not so much. I was already pondering about such optimizations for the
>>> plane gamma/csc stuff in my vblank branch.
>> Ok, so I can submit a v2 based on dinq with only the
>> 	if (!old_crtc_state->base.gamma_lut && new_crtc_state->base.gamma_lut)
>> check, or
>>> But for the fastboot case I think what we could do is just sanitize
>>> the LUT(s) after readout if gamma wasn't enabled by the BIOS.
>> We could do this, but this falls a bit outside of my expertise, I would be
>> more then happy to test a patch on one of the machines which needs this
>> LUTS sanitizing though. I get a very visible flash of a couple of bright
>> blue or yellow (2 different machines) on the upper few lines the first
>> time the gamma table gets loaded, so verifying that the sanitation kicks
>> in is easy.
> Actually, how recent is your kernel?
> Some issues with the gamma readout were fixed quite recently:
> 9b000b47cc18 ("drm/i915/color: fix broken gamma state-checker during boot")
> d50341274d01 ("drm/i915/color: move check of gamma_enable to specific func/platform")

I was testing with 5.4-rc# which does not have these commits. I've switched
to dinq but that does not help.

> If those don't help we could try the quick path and just blast the
> gamma from orbit like so:
> @@ -16787,18 +16787,23 @@ static int intel_initial_commit(struct drm_device *dev)
>   			goto out;
>   		}
> +		/*
> +		 * Let's just turn off the BIOS leftover LUT(s).
> +		 *
> +		 * FIXME maybe we shouldn't do this, and instead
> +		 * we should let fb_helper/whatever replace the
> +		 * LUT(s) when they start to actually render stuff.
> +		 * But for now we may get an ugly flash due to
> +		 * non-atomic gamma updates.
> +		 */
> +		drm_property_replace_blob(&crtc_state->degamma_lut, NULL);
> +		drm_property_replace_blob(&crtc_state->gamma_lut, NULL);
> +		crtc_state->color_mgmt_changed = true;
> +
>   		if (crtc_state->active) {
>   			ret = drm_atomic_add_affected_planes(state, crtc);
>   			if (ret)
>   				goto out;
> -
> -			/*
> -			 * FIXME hack to force a LUT update to avoid the
> -			 * plane update forcing the pipe gamma on without
> -			 * having a proper LUT loaded. Remove once we
> -			 * have readout for pipe gamma enable.
> -			 */
> -			crtc_state->color_mgmt_changed = true;
>   		}
>   	}

Ok, I've given this a try, but this also does not help, which is somewhat
expected. We only load the LUT when crtc_state->gamma_lut != NULL so
I believe that explicitly setting to NULL from intel_initial_commit() does
not make a difference, as the PIPECONF_GAMMA_MODE bits in the PIPECONF register
are already set to disabled.

If anything we should force load a linear gamma table to the pipe's LUT at
probe time when gamma was not enabled by the GOP, rather then setting the
gamma_lut to NULL.

My theory is that the LUT is *not* used by the GOP so crtc_state->gamma_enable
is initially false. The problem happens when userspace then actually sets
the gamma_lut blob. My theory is as follows:

1. GOP initializes DSI panel on CHT, does not use the "legacy gamma LUT"
2. i915 driver loads, all is well
3. plymouth does a drmModeCrtcSetGamma
4. intel_color.c sets crtc_state->gamma_enable to true on the commit
5. Since commit 051a6d8d3ca0 ("drm/i915: Move LUT programming to happen after vblank waits")
    we program the double-bufferred PIPECONF register which has the legacy gamma enable bit
    before the vblank;
6. While we upload the new legacy gamma LUT values after vblank.
7. This causes the first few lines of the frame after the vblank to use the old LUT
    values and the old values appear to contain noise (this seems to be the same
    noise every boot given a certain model hw). This noise leads to a bright blue or yellow
    (depending on the hw) flash in the first couple of lines.

I will submit a new version of my patch checking only gamma_enable as you requested,
then we'll see from there.



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