Re: [PATCH v4] drm/i915/dsi: let HW maintain the HS-TRAIL timing

[Jani Nikula <jani.nikula@linux.intel.com>]

On Tue, 11 Mar 2025, William Tseng <william.tseng@intel.com> wrote:
> This change is to avoid over-specification of the TEOT timing
> parameter, which is derived from software in current design.
>
> Supposed that THS-TRAIL and THS-EXIT have the minimum values,
> i.e., 60 and 100 in ns. If SW is overriding the HW default,
> the TEOT value becomes 150 ns, approximately calculated by
> the following formula.
>
>   DIV_ROUND_UP(60/50)*50 + DIV_ROUND_UP(100/50))*50/2, where 50
>   is LP Escape Clock time in ns.
>
> The TEOT value 150 ns is larger than the maximum value,
> around 136 ns if UI is 1.8ns, (105 ns + 12*UI, defined by MIPI
> DPHY specification).
>
> However, the TEOT value will meet the specification if THS-TRAIL
> is set to the HW default, instead of software overriding.
>
> The timing change is made for both data lane and clock lane.

What does the VBT contain?

Granted, the spec only describes the relevant fields as "TClkTrail:
Clock Trail value" and "THSTrail: HS Trail value".

But we might have panels and setups where we depend on VBT providing the
values. So why do we pick and choose some VBT values for overrides and
not others?

I also don't get why we do this:

	tclk_trail_ns = max(mipi_config->tclk_trail, mipi_config->ths_trail);

and then use that for both. Again, would be interesting to see what VBT
has for them.


BR,
Jani.


>
> v1: initial version.
> v2: change clock lane dphy timings.
> v3: remove calculation of trail cnt.
> v4: rebase.
>
> Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/13891
> Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
> Cc: Jani Nikula <jani.nikula@linux.intel.com>
> Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
> Cc: Lee Shawn C <shawn.c.lee@intel.com>
> Cc: Cooper Chiou <cooper.chiou@intel.com>
> Signed-off-by: William Tseng <william.tseng@intel.com>
> Acked-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
> ---
>  drivers/gpu/drm/i915/display/icl_dsi.c | 19 +++----------------
>  1 file changed, 3 insertions(+), 16 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/display/icl_dsi.c b/drivers/gpu/drm/i915/display/icl_dsi.c
> index 82bf6c654de2..6cf9c9275031 100644
> --- a/drivers/gpu/drm/i915/display/icl_dsi.c
> +++ b/drivers/gpu/drm/i915/display/icl_dsi.c
> @@ -1846,14 +1846,13 @@ static void icl_dphy_param_init(struct intel_dsi *intel_dsi)
>  	struct intel_connector *connector = intel_dsi->attached_connector;
>  	struct mipi_config *mipi_config = connector->panel.vbt.dsi.config;
>  	u32 tlpx_ns;
> -	u32 prepare_cnt, exit_zero_cnt, clk_zero_cnt, trail_cnt;
> -	u32 ths_prepare_ns, tclk_trail_ns;
> +	u32 prepare_cnt, exit_zero_cnt, clk_zero_cnt;
> +	u32 ths_prepare_ns;
>  	u32 hs_zero_cnt;
>  	u32 tclk_pre_cnt;
>  
>  	tlpx_ns = intel_dsi_tlpx_ns(intel_dsi);
>  
> -	tclk_trail_ns = max(mipi_config->tclk_trail, mipi_config->ths_trail);
>  	ths_prepare_ns = max(mipi_config->ths_prepare,
>  			     mipi_config->tclk_prepare);
>  
> @@ -1880,14 +1879,6 @@ static void icl_dphy_param_init(struct intel_dsi *intel_dsi)
>  		clk_zero_cnt = ICL_CLK_ZERO_CNT_MAX;
>  	}
>  
> -	/* trail cnt in escape clocks*/
> -	trail_cnt = DIV_ROUND_UP(tclk_trail_ns, tlpx_ns);
> -	if (trail_cnt > ICL_TRAIL_CNT_MAX) {
> -		drm_dbg_kms(display->drm, "trail_cnt out of range (%d)\n",
> -			    trail_cnt);
> -		trail_cnt = ICL_TRAIL_CNT_MAX;
> -	}
> -
>  	/* tclk pre count in escape clocks */
>  	tclk_pre_cnt = DIV_ROUND_UP(mipi_config->tclk_pre, tlpx_ns);
>  	if (tclk_pre_cnt > ICL_TCLK_PRE_CNT_MAX) {
> @@ -1920,17 +1911,13 @@ static void icl_dphy_param_init(struct intel_dsi *intel_dsi)
>  			       CLK_ZERO_OVERRIDE |
>  			       CLK_ZERO(clk_zero_cnt) |
>  			       CLK_PRE_OVERRIDE |
> -			       CLK_PRE(tclk_pre_cnt) |
> -			       CLK_TRAIL_OVERRIDE |
> -			       CLK_TRAIL(trail_cnt));
> +			       CLK_PRE(tclk_pre_cnt));
>  
>  	/* data lanes dphy timings */
>  	intel_dsi->dphy_data_lane_reg = (HS_PREPARE_OVERRIDE |
>  					 HS_PREPARE(prepare_cnt) |
>  					 HS_ZERO_OVERRIDE |
>  					 HS_ZERO(hs_zero_cnt) |
> -					 HS_TRAIL_OVERRIDE |
> -					 HS_TRAIL(trail_cnt) |
>  					 HS_EXIT_OVERRIDE |
>  					 HS_EXIT(exit_zero_cnt));

-- 
Jani Nikula, Intel