Re: [PATCH 2/3] drm/i915/ltphy: Compare only certain fields in state verify function

["Nautiyal, Ankit K" <ankit.k.nautiyal@intel.com>]

On 12/31/2025 10:53 AM, Suraj Kandpal wrote:
> Verify only the config[0,2] fields in the LT PHY state since these
> are the only reliable values we can get back when we read the VDR
> registers. The reason being that the state does not persist for other
> VDR registers when power gating comes into picture.
> Though not ideal this change does not hit us badly in perspective of how
> we use the compare function to decide if fastset is required or if we
> wrote the state correctly. VDR0_CONFIG and VDR1_CONFIG hold the values
> that indicate the PLL operating mode and link rate which is usually
> what we need to check if something has changed or not.
>
> Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
> ---
>   drivers/gpu/drm/i915/display/intel_lt_phy.c | 17 +++++++++++------
>   1 file changed, 11 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_lt_phy.c b/drivers/gpu/drm/i915/display/intel_lt_phy.c
> index 9501ac861712..cc85818c2b7e 100644
> --- a/drivers/gpu/drm/i915/display/intel_lt_phy.c
> +++ b/drivers/gpu/drm/i915/display/intel_lt_phy.c
> @@ -2207,13 +2207,18 @@ bool
>   intel_lt_phy_pll_compare_hw_state(const struct intel_lt_phy_pll_state *a,
>   				  const struct intel_lt_phy_pll_state *b)
>   {
> -	if (memcmp(&a->config, &b->config, sizeof(a->config)) != 0)
> -		return false;
> -
> -	if (memcmp(&a->data, &b->data, sizeof(a->data)) != 0)
> -		return false;
> +	/*
> +	 * With LT PHY values other than VDR0_CONFIG and VDR2_CONFIG are
> +	 * unreliable and cannot always be read back since internally
> +	 * the after power gating values are not restored back to the
> +	 * shadow VDR registers, hence we do not compare the whole state
> +	 * just the two VDR registers.

This can be broken down into simpler sentences.

With above fixed:


Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>


> +	 */
> +	if (a->config[0] == b->config[0] &&
> +	    a->config[2] == b->config[2])
> +		return true;
>   
> -	return true;
> +	return false;
>   }
>   
>   void intel_lt_phy_pll_readout_hw_state(struct intel_encoder *encoder,