[PATCHv2] drm/sun4i: validate modes for HDMI

[Hans Verkuil]

On 13/12/17 11:30, Maxime Ripard wrote:
> Hi Hans,
> 
> On Fri, Dec 08, 2017 at 04:48:47PM +0100, Hans Verkuil wrote:
>> When I connected my cubieboard running 4.15-rc1 to my 4k display I got no picture. Some
>> digging found that there is no check against the upper pixelclock limit of the HDMI
>> output, so X selects a 4kp60 format at 594 MHz, which obviously won't work.
>>
>> The patch below adds a check for the upper bound of what this hardware can do, and
>> it checks if the requested tmds clock can be obtained.
>>
>> Signed-off-by: Hans Verkuil <hans.verkuil at cisco.com>
>> ---
>>  drivers/gpu/drm/sun4i/sun4i_hdmi_enc.c | 19 +++++++++++++++++++
>>  1 file changed, 19 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/sun4i/sun4i_hdmi_enc.c b/drivers/gpu/drm/sun4i/sun4i_hdmi_enc.c
>> index dda904ec0534..c10400a19b33 100644
>> --- a/drivers/gpu/drm/sun4i/sun4i_hdmi_enc.c
>> +++ b/drivers/gpu/drm/sun4i/sun4i_hdmi_enc.c
>> @@ -208,8 +208,27 @@ static int sun4i_hdmi_get_modes(struct drm_connector *connector)
>>  	return ret;
>>  }
>>
>> +static int sun4i_hdmi_mode_valid(struct drm_connector *connector,
>> +				 struct drm_display_mode *mode)
>> +{
>> +	struct sun4i_hdmi *hdmi = drm_connector_to_sun4i_hdmi(connector);
>> +	unsigned long rate = mode->clock * 1000;
>> +	long rounded_rate;
>> +
>> +	/* 165 MHz is the typical max pixelclock frequency for HDMI <= 1.2 */
>> +	if (rate > 165000000)
>> +		return MODE_CLOCK_HIGH;
>> +	rounded_rate = clk_round_rate(hdmi->tmds_clk, rate);
>> +	if (rounded_rate < rate)
>> +		return MODE_CLOCK_LOW;
>> +	if (rounded_rate > rate)
>> +		return MODE_CLOCK_HIGH;
>> +	return MODE_OK;
>> +}
> 
> This looks much better, thanks!
> 
> One thing that I was mentionning in my other mail is that our rate
> rounding might not provide the exact TMDS clock rate advertised by the
> EDID, while staying in the tolerancy.
> 
> We've raised this issue before, without coming to a conclusion. Would
> you happen to know what that tolerancy would be on an HDMI link?

I can't actually find anything about that in the HDMI spec. However, the VESA DMT
spec specifies a tolerance of +/- 0.5% for the pixelclock.

The HDMI 2.1 spec also suggests that this is the tolerance (caveat: I have not had
the time to study this in detail, but it does mention it when describing the new
variable refresh rate feature).

That said, the problem with a 0.5% tolerance is that the slight slowdown for 59.94
vs 60 Hz framerate falls within that tolerance (it's a 0.1% slowdown).

Generally clocks will be able to hit the standard frequencies (74.25, 148.5, etc)
exactly, but if you want to slow down for 59.94 framerate they tend to be off by
a bit.

In the end I think keeping a margin of 0.4 or 0.5% is the best approach.

Regards,

	Hans