linux-kernel - Re: [PATCH] drm/omap: increase DSS5 max tv pclk to 192MHz

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Date:   Wed, 10 Nov 2021 12:22:18 +0200
From:   Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
To:     Neil Armstrong <narmstrong@...libre.com>
Cc:     linux-omap@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, khilman@...libre.com,
        Tomi Valkeinen <tomi.valkeinen@...com>
Subject: Re: [PATCH] drm/omap: increase DSS5 max tv pclk to 192MHz

On 09/11/2021 15:33, Neil Armstrong wrote:
> Hi Tomi,
> 
> On 12/10/2021 15:39, Neil Armstrong wrote:
>> From: Tomi Valkeinen <tomi.valkeinen@...com>
>>
>> DSS5's maximum tv pclk rate (i.e. HDMI) is set to 186MHz, which comes
>> from the TRM (DPLL_HDMI_CLK1 frequency must be lower than 186 MHz). To
>> support DRA76's wide screen HDMI feature, we need to increase this
>> maximum rate.
>>
>> Testing shows that the PLL seems to work fine even with ~240MHz clocks,
>> and even the HDMI output at that clock is stable enough for monitors to
>> show a picture. This holds true for all DRA7 and AM5 SoCs (and probably
>> also for OMAP5).
>>
>> However, the highest we can go without big refactoring to the clocking
>> code is 192MHz, as that is the DSS func clock we get from the PRCM. So,
>> increase the max HDMI pixel clock to 192MHz for now, to allow some more
>> 2k+ modes to work.
>>
>> This patch never had a clear confirmation from HW people, but this
>> change stayed on production trees for multiple years without any report
>> on an eventual breakage.
>>
>> Signed-off-by: Tomi Valkeinen <tomi.valkeinen@...com>
>> Signed-off-by: Neil Armstrong <narmstrong@...libre.com>
>> ---
>>   drivers/gpu/drm/omapdrm/dss/dispc.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> Tomi,
>>
>> I slighly changed the commit message to point the fact this patch has been
>> used in production fort years without any sign of breakage.
>>
>> Neil
>>
>> diff --git a/drivers/gpu/drm/omapdrm/dss/dispc.c b/drivers/gpu/drm/omapdrm/dss/dispc.c
>> index 5619420cc2cc..3c4a4991e45a 100644
>> --- a/drivers/gpu/drm/omapdrm/dss/dispc.c
>> +++ b/drivers/gpu/drm/omapdrm/dss/dispc.c
>> @@ -4458,7 +4458,7 @@ static const struct dispc_features omap54xx_dispc_feats = {
>>   	.mgr_width_max		=	4096,
>>   	.mgr_height_max		=	4096,
>>   	.max_lcd_pclk		=	170000000,
>> -	.max_tv_pclk		=	186000000,
>> +	.max_tv_pclk		=	192000000,
>>   	.max_downscale		=	4,
>>   	.max_line_width		=	2048,
>>   	.min_pcd		=	1,
>>
>> base-commit: e4e737bb5c170df6135a127739a9e6148ee3da82
>>
> 
> Gentle ping,

Thanks, I'll apply this to drm-misc-next.

  Tomi