Re: [PATCH 1/7] drm/i915/dp: Fix 2.7 Gbps DP_LINK_BW value on g4x

[Imre Deak <imre.deak@intel.com>]

On Thu, Jul 10, 2025 at 11:17:12PM +0300, Ville Syrjala wrote:
> From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> 
> On g4x we currently use the 96MHz non-SSC refclk, which can't actually
> generate an exact 2.7 Gbps link rate. In practice we end up with 2.688
> Gbps which seems to be close enough to actually work, but link training
> is currently failing due to miscalculating the DP_LINK_BW value (we
> calcualte it directly from port_clock which reflects the actual PLL
> outpout frequency).
> 
> Ideas how to fix this:
> - nudge port_clock back up to 270000 during PLL computation/readout
> - track port_clock and the nominal link rate separately so they might
>   differ a bit
> - switch to the 100MHz refclk, but that one should be SSC so perhaps
>   not something we want
> 
> While we ponder about a better solution apply some band aid to the
> immediate issue of miscalculated DP_LINK_BW value. With this
> I can again use 2.7 Gbps link rate on g4x.
> 
> Cc: stable@vger.kernel.org
> Fixes: 665a7b04092c ("drm/i915: Feed the DPLL output freq back into crtc_state")
> Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

Reviewed-by: Imre Deak <imre.deak@intel.com>

IIUC, port_clock for g4x is ref * m / n / p, where for DP the fixed
ref=96000 and m/n/p values from g4x_dpll are used.

Ftr, m = 135, n = 6, p = 8 would give port_clock = 270000, but there's
no intel_limit for DP, so can't know if these params are within range.

> ---
>  drivers/gpu/drm/i915/display/intel_dp.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
> index f48912f308df..7976fec88606 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> @@ -1606,6 +1606,12 @@ int intel_dp_rate_select(struct intel_dp *intel_dp, int rate)
>  void intel_dp_compute_rate(struct intel_dp *intel_dp, int port_clock,
>  			   u8 *link_bw, u8 *rate_select)
>  {
> +	struct intel_display *display = to_intel_display(intel_dp);
> +
> +	/* FIXME g4x can't generate an exact 2.7GHz with the 96MHz non-SSC refclk */
> +	if (display->platform.g4x && port_clock == 268800)
> +		port_clock = 270000;
> +
>  	/* eDP 1.4 rate select method. */
>  	if (intel_dp->use_rate_select) {
>  		*link_bw = 0;
> -- 
> 2.49.0
>