Re: [PATCH] drm: bridge: fsl-ldb: fixup mode on freq mismatch

[Luca Ceresoli <luca.ceresoli@bootlin.com>]

+Cc: Miquèl, who is actively working on imx8mp video clock rates.

On Tue, 26 Nov 2024 16:45:54 +0100
Nikolaus Voss <nv@vosn.de> wrote:

> LDB clock has to be a fixed multiple of the pixel clock.
> As LDB and pixel clock are derived from different clock sources
> (at least on imx8mp), this constraint cannot be satisfied for
> any pixel clock, which leads to flickering and incomplete
> lines on the attached display.
> 
> To overcome this, check this condition in mode_fixup() and
> adapt the pixel clock accordingly.
> 
> Cc: <stable@vger.kernel.org>
> 
> Signed-off-by: Nikolaus Voss <nv@vosn.de>
> ---
>  drivers/gpu/drm/bridge/fsl-ldb.c | 40 ++++++++++++++++++++++++++++----
>  1 file changed, 36 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/bridge/fsl-ldb.c b/drivers/gpu/drm/bridge/fsl-ldb.c
> index 0e4bac7dd04ff..e341341b8c600 100644
> --- a/drivers/gpu/drm/bridge/fsl-ldb.c
> +++ b/drivers/gpu/drm/bridge/fsl-ldb.c
> @@ -104,12 +104,14 @@ static inline struct fsl_ldb *to_fsl_ldb(struct drm_bridge *bridge)
>  	return container_of(bridge, struct fsl_ldb, bridge);
>  }
>  
> +static unsigned int fsl_ldb_link_freq_factor(const struct fsl_ldb *fsl_ldb)
> +{
> +	return fsl_ldb_is_dual(fsl_ldb) ? 3500 : 7000;
> +}
> +
>  static unsigned long fsl_ldb_link_frequency(struct fsl_ldb *fsl_ldb, int clock)
>  {
> -	if (fsl_ldb_is_dual(fsl_ldb))
> -		return clock * 3500;
> -	else
> -		return clock * 7000;
> +	return clock * fsl_ldb_link_freq_factor(fsl_ldb);
>  }
>  
>  static int fsl_ldb_attach(struct drm_bridge *bridge,
> @@ -121,6 +123,35 @@ static int fsl_ldb_attach(struct drm_bridge *bridge,
>  				 bridge, flags);
>  }
>  
> +static bool fsl_ldb_mode_fixup(struct drm_bridge *bridge,
> +				const struct drm_display_mode *mode,
> +				struct drm_display_mode *adjusted_mode)
> +{
> +	const struct fsl_ldb *fsl_ldb = to_fsl_ldb(bridge);
> +	unsigned long requested_link_freq =
> +		mode->clock * fsl_ldb_link_freq_factor(fsl_ldb);
> +	unsigned long freq = clk_round_rate(fsl_ldb->clk, requested_link_freq);
> +
> +	if (freq != requested_link_freq) {
> +		/*
> +		 * this will lead to flicker and incomplete lines on
> +		 * the attached display, adjust the CRTC clock
> +		 * accordingly.
> +		 */
> +		int pclk = freq / fsl_ldb_link_freq_factor(fsl_ldb);
> +
> +		if (adjusted_mode->clock != pclk) {
> +			dev_warn(fsl_ldb->dev, "Adjusted pixel clk to match LDB clk (%d kHz -> %d kHz)!\n",
> +				 adjusted_mode->clock, pclk);
> +
> +			adjusted_mode->clock = pclk;
> +			adjusted_mode->crtc_clock = pclk;
> +		}
> +	}
> +
> +	return true;
> +}
> +
>  static void fsl_ldb_atomic_enable(struct drm_bridge *bridge,
>  				  struct drm_bridge_state *old_bridge_state)
>  {
> @@ -280,6 +311,7 @@ fsl_ldb_mode_valid(struct drm_bridge *bridge,
>  
>  static const struct drm_bridge_funcs funcs = {
>  	.attach = fsl_ldb_attach,
> +	.mode_fixup = fsl_ldb_mode_fixup,
>  	.atomic_enable = fsl_ldb_atomic_enable,
>  	.atomic_disable = fsl_ldb_atomic_disable,
>  	.atomic_duplicate_state = drm_atomic_helper_bridge_duplicate_state,



-- 
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com