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

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Message-ID: <b0683f52-abde-cb11-c88c-dd05645945a0@baylibre.com>
Date:   Tue, 9 Nov 2021 14:33:31 +0100
From:   Neil Armstrong <narmstrong@...libre.com>
To:     tomba@...nel.org
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

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,

Neil