[PATCH 4/4] drm/i915/intel_dsi: Read back pclk set by GOP and use that as pclk

[Hans de Goede]

Hi,

On 07/06/2018 04:16 PM, Ville Syrjälä wrote:
> On Tue, Jun 19, 2018 at 10:18:27PM +0200, Hans de Goede wrote:
>> On BYT and CHT the GOP sometimes initializes the pclk at a (slightly)
>> different frequency then the pclk which we've calculated.
>>
>> This commit makes the DSI code read-back the pclk set by the GOP and
>> if that is within a reasonable margin of the calculated pclk, uses
>> that instead.
>>
>> This fixes the first modeset being a full modeset instead of a
>> fast modeset on systems where the GOP pclk is different.
>>
>> Signed-off-by: Hans de Goede <hdegoede at redhat.com>
>> ---
>>   drivers/gpu/drm/i915/intel_dsi_vbt.c | 14 ++++++++++++++
>>   1 file changed, 14 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/i915/intel_dsi_vbt.c b/drivers/gpu/drm/i915/intel_dsi_vbt.c
>> index 4d6ffa7b3e7b..d4cc6099012c 100644
>> --- a/drivers/gpu/drm/i915/intel_dsi_vbt.c
>> +++ b/drivers/gpu/drm/i915/intel_dsi_vbt.c
>> @@ -517,6 +517,7 @@ bool intel_dsi_vbt_init(struct intel_dsi *intel_dsi, u16 panel_id)
>>   	u32 mul;
>>   	u16 burst_mode_ratio;
>>   	enum port port;
>> +	enum pipe pipe;
>>   
>>   	DRM_DEBUG_KMS("\n");
>>   
>> @@ -583,6 +584,19 @@ bool intel_dsi_vbt_init(struct intel_dsi *intel_dsi, u16 panel_id)
>>   	} else
>>   		burst_mode_ratio = 100;
>>   
>> +	/*
>> +	 * On BYT / CRC the GOP sometimes picks a slightly different pclk,
>> +	 * read back the GOP configured pclk and prefer it over ours.
>> +	 */
>> +	if ((IS_VALLEYVIEW(dev_priv) || IS_CHERRYVIEW(dev_priv)) &&
>> +	    intel_dsi_get_hw_state(&intel_dsi->base, &pipe)) {
>> +		u32 gop = intel_dsi_get_pclk(&intel_dsi->base, bpp, NULL);
>> +
>> +		DRM_DEBUG_KMS("Calculated pclk %d GOP %d\n", pclk, gop);
>> +		if (gop >= (pclk * 9 / 10) && gop <= (pclk * 11 / 10))
>> +			pclk = gop;
>> +	}
> 
> Is the gop acually picking a different clock that what we pick in the
> end, or is it just that the value in the vbt doesn't quite match what we
> (and the gop) end up using on account of limitations of the pll?

I *think* the GOP is picking a different clock, IIRC (*) it is something like
90Mhz for the GOP and the VBT says 87Mhz (and our calculations leave it
unmodified. With this patch which puts pclk at 90Mhz on the specific
tablet I developed this on, the PLL settings calculated by our PLL code
end up being exactly the same as the once chosen by the GOP once we have
the pclk set to 90MHz.

Note I've seen these small (and sometimes somewhat bigger) differences
between GOP and VBT pclk on a lot of devices, not just the one tablet
I developed it on. Since submitting this I've run this on at least
5 different CHT/BYT devices and it works as advertised so far.

> For that particular problem I think I had patches long ago to go through
> the pll computation during init so that we basically fix up the slightly
> bogus clock from the vbt.

We do end up with a slightly different clock then the 87MHhz when going
though the PLL calculations, something like 86.33MHz or some such from
the top of my head, but the problem is not the pclk not matching the
intel_pipe_config_compare() function does not look at it, it looks at
dsi_pll.ctrl dsi_pll.div and those don't match, where as they do match
if we fixup the VBT clock to be the one confgured by the GOP.

> Any kind of hack that involves reading out the hardware state should go
> into something like intel_sanitize_encoder(). Actually by that time we
> have already read out the hw state, so it shouldn't require any
> modifications to the existing dsi code itself.

I do not think that intel_sanitize encoder is the right place to do this:

* I don't want to modify the read-back state, I want to modify our
   calculated "new/ideal" state to match the read-back state
* I don't want to directly modify our calculated new/ideal state,
   instead I want to cleanup / sanitize the values we got from the VBT
   so that the initial-modeset *and* any future modesets will use the
   GOP pclk. I believe it is important that if we're going to use the
   GOP pclk we use it for all modesets consistently.
* I only want to do this once, at boot when we are sure the mode was
   set by the GOP and not after suspend/resume when we don't know if the
   GOP will have touched things or not.
* It is DSI specific, whereas sofar intel_sanitize_encoder seems to
   not contain any output specific code.

Regards,

Hans



> 
>> +
>>   	intel_dsi->burst_mode_ratio = burst_mode_ratio;
>>   	intel_dsi->pclk = pclk;
>>   
>> -- 
>> 2.17.1
>