Re: [PATCH v5 2/3] drm/tidss: Remove max_pclk_khz from tidss display features

[Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>]

Hi,

On 19/08/2025 22:21, Swamil Jain wrote:
> From: Jayesh Choudhary <j-choudhary@ti.com>
> 
> TIDSS hardware by itself does not have variable max_pclk for each VP.
> The maximum pixel clock is determined by the limiting factor between
> the functional clock and the PLL (parent to the VP/pixel clock).

Hmm, this is actually not in the driver, is it? We're not limiting the
pclk based on the fclk.

> The limitation that has been modeled till now comes from the clock
> (PLL can only be programmed to a particular max value). Instead of
> putting it as a constant field in dispc_features, we can query the
> DM to see if requested clock can be set or not and use it in
> mode_valid().
> 
> Replace constant "max_pclk_khz" in dispc_features with
> max_successful_rate and max_attempted_rate, both of these in
> tidss_device structure would be modified in runtime. In mode_valid()
> call, check if a best frequency match for mode clock can be found or
> not using "clk_round_rate()". Based on that, propagate
> max_successful_rate and max_attempted_rate and query DM again only if
> the requested mode clock is greater than max_attempted_rate. (As the
> preferred display mode is usually the max resolution, driver ends up
> checking the highest clock the first time itself which is used in
> subsequent checks).
> 
> Since TIDSS display controller provides clock tolerance of 5%, we use
> this while checking the max_successful_rate. Also, move up
> "dispc_pclk_diff()" before it is called.
> 
> This will make the existing compatibles reusable if DSS features are
> same across two SoCs with the only difference being the pixel clock.
> 
> Fixes: 7246e0929945 ("drm/tidss: Add OLDI bridge support")
> Reviewed-by: Devarsh Thakkar <devarsht@ti.com>
> Signed-off-by: Jayesh Choudhary <j-choudhary@ti.com>
> Signed-off-by: Swamil Jain <s-jain1@ti.com>
> ---
>  drivers/gpu/drm/tidss/tidss_dispc.c | 85 +++++++++++++----------------
>  drivers/gpu/drm/tidss/tidss_dispc.h |  1 -
>  drivers/gpu/drm/tidss/tidss_drv.h   | 11 +++-
>  3 files changed, 47 insertions(+), 50 deletions(-)
> 
> diff --git a/drivers/gpu/drm/tidss/tidss_dispc.c b/drivers/gpu/drm/tidss/tidss_dispc.c
> index c0277fa36425..c2c0fe0d4a0f 100644
> --- a/drivers/gpu/drm/tidss/tidss_dispc.c
> +++ b/drivers/gpu/drm/tidss/tidss_dispc.c
> @@ -58,10 +58,6 @@ static const u16 tidss_k2g_common_regs[DISPC_COMMON_REG_TABLE_LEN] = {
>  const struct dispc_features dispc_k2g_feats = {
>  	.min_pclk_khz = 4375,
>  
> -	.max_pclk_khz = {
> -		[DISPC_VP_DPI] = 150000,
> -	},
> -
>  	/*
>  	 * XXX According TRM the RGB input buffer width up to 2560 should
>  	 *     work on 3 taps, but in practice it only works up to 1280.
> @@ -144,11 +140,6 @@ static const u16 tidss_am65x_common_regs[DISPC_COMMON_REG_TABLE_LEN] = {
>  };
>  
>  const struct dispc_features dispc_am65x_feats = {
> -	.max_pclk_khz = {
> -		[DISPC_VP_DPI] = 165000,
> -		[DISPC_VP_OLDI_AM65X] = 165000,
> -	},
> -
>  	.scaling = {
>  		.in_width_max_5tap_rgb = 1280,
>  		.in_width_max_3tap_rgb = 2560,
> @@ -244,11 +235,6 @@ static const u16 tidss_j721e_common_regs[DISPC_COMMON_REG_TABLE_LEN] = {
>  };
>  
>  const struct dispc_features dispc_j721e_feats = {
> -	.max_pclk_khz = {
> -		[DISPC_VP_DPI] = 170000,
> -		[DISPC_VP_INTERNAL] = 600000,
> -	},
> -
>  	.scaling = {
>  		.in_width_max_5tap_rgb = 2048,
>  		.in_width_max_3tap_rgb = 4096,
> @@ -315,11 +301,6 @@ const struct dispc_features dispc_j721e_feats = {
>  };
>  
>  const struct dispc_features dispc_am625_feats = {
> -	.max_pclk_khz = {
> -		[DISPC_VP_DPI] = 165000,
> -		[DISPC_VP_INTERNAL] = 170000,
> -	},
> -
>  	.scaling = {
>  		.in_width_max_5tap_rgb = 1280,
>  		.in_width_max_3tap_rgb = 2560,
> @@ -376,15 +357,6 @@ const struct dispc_features dispc_am625_feats = {
>  };
>  
>  const struct dispc_features dispc_am62a7_feats = {
> -	/*
> -	 * if the code reaches dispc_mode_valid with VP1,
> -	 * it should return MODE_BAD.
> -	 */
> -	.max_pclk_khz = {
> -		[DISPC_VP_TIED_OFF] = 0,
> -		[DISPC_VP_DPI] = 165000,
> -	},
> -
>  	.scaling = {
>  		.in_width_max_5tap_rgb = 1280,
>  		.in_width_max_3tap_rgb = 2560,
> @@ -441,10 +413,6 @@ const struct dispc_features dispc_am62a7_feats = {
>  };
>  
>  const struct dispc_features dispc_am62l_feats = {
> -	.max_pclk_khz = {
> -		[DISPC_VP_DPI] = 165000,
> -	},
> -
>  	.subrev = DISPC_AM62L,
>  
>  	.common = "common",
> @@ -1347,25 +1315,57 @@ static void dispc_vp_set_default_color(struct dispc_device *dispc,
>  			DISPC_OVR_DEFAULT_COLOR2, (v >> 32) & 0xffff);
>  }
>  
> +/*
> + * Calculate the percentage difference between the requested pixel clock rate
> + * and the effective rate resulting from calculating the clock divider value.
> + */
> +unsigned int dispc_pclk_diff(unsigned long rate, unsigned long real_rate)
> +{
> +	int r = rate / 100, rr = real_rate / 100;
> +
> +	return (unsigned int)(abs(((rr - r) * 100) / r));
> +}
> +
> +static int check_pixel_clock(struct dispc_device *dispc,
> +			     u32 hw_videoport, unsigned long clock)
> +{
> +	unsigned long round_clock;
> +
> +	if (dispc->tidss->is_ext_vp_clk[hw_videoport])
> +		return 0;
> +
> +	if (clock <= dispc->tidss->max_successful_rate[hw_videoport])
> +		return 0;
> +
> +	if (clock < dispc->tidss->max_attempted_rate[hw_videoport])
> +		return -EINVAL;
> +
> +	round_clock = clk_round_rate(dispc->vp_clk[hw_videoport], clock);
> +
> +	if (dispc_pclk_diff(clock, round_clock) > 5)
> +		return -EINVAL;
> +
> +	dispc->tidss->max_successful_rate[hw_videoport] = round_clock;
> +	dispc->tidss->max_attempted_rate[hw_videoport] = clock;

I still don't think this logic is sound. This is trying to find the
maximum clock rate, and optimize by avoiding the calls to
clk_round_rate() if possible. That makes sense.

But checking for the 5% tolerance breaks it, in my opinion. If we find
out that the PLL can do, say, 100M, but we need pclk of 90M, the current
maximum is still the 100M, isn't it?

Why can't we replace the "if (mode->clock > max_pclk)" check with a new
check that only looks for the max rate? If we want to add tolerance
checks to mode_valid (which are currently not there), let's add it in a
separate patch.

 Tomi

> +	return 0;
> +}
> +
>  enum drm_mode_status dispc_vp_mode_valid(struct dispc_device *dispc,
>  					 u32 hw_videoport,
>  					 const struct drm_display_mode *mode)
>  {
>  	u32 hsw, hfp, hbp, vsw, vfp, vbp;
>  	enum dispc_vp_bus_type bus_type;
> -	int max_pclk;
>  
>  	bus_type = dispc->feat->vp_bus_type[hw_videoport];
>  
> -	max_pclk = dispc->feat->max_pclk_khz[bus_type];
> -
> -	if (WARN_ON(max_pclk == 0))
> +	if (WARN_ON(bus_type == DISPC_VP_TIED_OFF))
>  		return MODE_BAD;
>  
>  	if (mode->clock < dispc->feat->min_pclk_khz)
>  		return MODE_CLOCK_LOW;
>  
> -	if (mode->clock > max_pclk)
> +	if (check_pixel_clock(dispc, hw_videoport, mode->clock * 1000))
>  		return MODE_CLOCK_HIGH;
>  
>  	if (mode->hdisplay > 4096)
> @@ -1437,17 +1437,6 @@ void dispc_vp_disable_clk(struct dispc_device *dispc, u32 hw_videoport)
>  	clk_disable_unprepare(dispc->vp_clk[hw_videoport]);
>  }
>  
> -/*
> - * Calculate the percentage difference between the requested pixel clock rate
> - * and the effective rate resulting from calculating the clock divider value.
> - */
> -unsigned int dispc_pclk_diff(unsigned long rate, unsigned long real_rate)
> -{
> -	int r = rate / 100, rr = real_rate / 100;
> -
> -	return (unsigned int)(abs(((rr - r) * 100) / r));
> -}
> -
>  int dispc_vp_set_clk_rate(struct dispc_device *dispc, u32 hw_videoport,
>  			  unsigned long rate)
>  {
> diff --git a/drivers/gpu/drm/tidss/tidss_dispc.h b/drivers/gpu/drm/tidss/tidss_dispc.h
> index b8614f62186c..45b1a8aa9089 100644
> --- a/drivers/gpu/drm/tidss/tidss_dispc.h
> +++ b/drivers/gpu/drm/tidss/tidss_dispc.h
> @@ -75,7 +75,6 @@ enum dispc_dss_subrevision {
>  
>  struct dispc_features {
>  	int min_pclk_khz;
> -	int max_pclk_khz[DISPC_VP_MAX_BUS_TYPE];
>  
>  	struct dispc_features_scaling scaling;
>  
> diff --git a/drivers/gpu/drm/tidss/tidss_drv.h b/drivers/gpu/drm/tidss/tidss_drv.h
> index 4e38cfa99e84..667c0d772519 100644
> --- a/drivers/gpu/drm/tidss/tidss_drv.h
> +++ b/drivers/gpu/drm/tidss/tidss_drv.h
> @@ -23,7 +23,16 @@ struct tidss_device {
>  	const struct dispc_features *feat;
>  	struct dispc_device *dispc;
>  	bool is_ext_vp_clk[TIDSS_MAX_PORTS];
> -
> +	/*
> +	 * Stores highest pixel clock value found to be valid while checking
> +	 * supported modes for connected display
> +	 */
> +	unsigned long max_successful_rate[TIDSS_MAX_PORTS];
> +	/*
> +	 * Stores the highest attempted pixel clock rate whose validated
> +	 * clock is within the tolerance range
> +	 */
> +	unsigned long max_attempted_rate[TIDSS_MAX_PORTS];
>  
>  	unsigned int num_crtcs;
>  	struct drm_crtc *crtcs[TIDSS_MAX_PORTS];