linux-kernel - Re: [PATCH 1/1] drm/bridge: tc358775: Remove burst mode support

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Message-ID: <7ca35f75-3423-4ed3-bce0-6f3c7fa1f718@linaro.org>
Date: Mon, 18 Nov 2024 09:43:56 +0100
From: Neil Armstrong <neil.armstrong@...aro.org>
To: Daniel Semkowicz <dse@...umatec.com>,
 Andrzej Hajda <andrzej.hajda@...el.com>, Robert Foss <rfoss@...nel.org>
Cc: David Airlie <airlied@...il.com>,
 Jernej Skrabec <jernej.skrabec@...il.com>, Jonas Karlman <jonas@...boo.se>,
 Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Michael Walle <mwalle@...nel.org>,
 Simona Vetter <simona@...ll.ch>, Thomas Zimmermann <tzimmermann@...e.de>,
 Tony Lindgren <tony@...mide.com>, dri-devel@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] drm/bridge: tc358775: Remove burst mode support

On 15/11/2024 17:06, Daniel Semkowicz wrote:
> Current tc358775 bridge driver implementation assumes that DSI clock
> is used as a source for LVDS pixel clock. Moreover, clock divider
> PCLKDIV has a hard-coded value. This means that driver expects DSI
> clock to always match a panel pixel clock. With such assumption, LVDS
> pixel clock is configured to be (DSI pixel clock / 3) for single link.
> This is true for DSI non-burst video modes. In burst mode, DSI host
> is allowed to set transmission rate higher than pixel clock to allow
> shorter data bursts. When the transmission rate is higher than expected
> by the bridge, LVDS output timings are still configured for the lower
> frequency, but the output is clocked with the higher one. In most cases,
> bad LVDS timings cause incorrect panel operation.
> 
> Remove support for burst mode, so the non-burst mode is used by DSI
> host by default. Burst mode is supported by the bridge itself,
> but requires proper implementation in the driver to operate correctly
> in all scenarios.
> 
> Fixes: a4ed72e85c46 ("drm/bridge: tc358775: Add burst and low-power modes")
> Signed-off-by: Daniel Semkowicz <dse@...umatec.com>
> ---
> 
>   drivers/gpu/drm/bridge/tc358775.c | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/bridge/tc358775.c b/drivers/gpu/drm/bridge/tc358775.c
> index 0b4efaca6d682..7496681c7b883 100644
> --- a/drivers/gpu/drm/bridge/tc358775.c
> +++ b/drivers/gpu/drm/bridge/tc358775.c
> @@ -632,8 +632,7 @@ static int tc_attach_host(struct tc_data *tc)
>   
>   	dsi->lanes = tc->num_dsi_lanes;
>   	dsi->format = MIPI_DSI_FMT_RGB888;
> -	dsi->mode_flags = MIPI_DSI_MODE_VIDEO | MIPI_DSI_MODE_VIDEO_BURST |
> -			  MIPI_DSI_MODE_LPM;
> +	dsi->mode_flags = MIPI_DSI_MODE_VIDEO | MIPI_DSI_MODE_LPM;
>   
>   	/*
>   	 * The hs_rate and lp_rate are data rate values. The HS mode is

Reviewed-by: Neil Armstrong <neil.armstrong@...aro.org>