Re: [PATCH 1/4] drm/i915/display: Limit max joined dotclock on joiner pipes available

On 8/7/2026 6:04 PM, Jani Nikula wrote:
> Multiplying the max dotclock based on joiner availability on the
> platform alone does not take into account the actual pipes (possibly
> with some fused) available for joining.
>
> Limit the max joined dotclock based on the actual pipes available for
> joining. Add helpers for figuring out the valid primary pipes, based on
> having the required joiners and amount of consecutive pipes available.
>
> v2: Check for consecutive pipes available (Ankit)
>
> Cc: Ankit Nautiyal <[email protected]>
> Signed-off-by: Jani Nikula <[email protected]>
> ---
>   drivers/gpu/drm/i915/display/intel_display.c | 51 +++++++++++++++++++-
>   drivers/gpu/drm/i915/display/intel_display.h |  1 +
>   2 files changed, 50 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_display.c b/drivers/gpu/drm/i915/display/intel_display.c
> index 829d7a411dcc..c958b2a92cc7 100644
> --- a/drivers/gpu/drm/i915/display/intel_display.c
> +++ b/drivers/gpu/drm/i915/display/intel_display.c
> @@ -193,6 +193,53 @@ is_trans_port_sync_mode(const struct intel_crtc_state *crtc_state)
>   		is_trans_port_sync_slave(crtc_state);
>   }
>   
> +/*
> + * Return a bitmask of all the start indices of consecutive bitfields of size
> + * width in mask.
> + */
> +static unsigned long find_consecutive_bits(unsigned long mask, int width)
> +{
> +	unsigned long bit, out_mask = 0;
> +
> +	if (!width)
> +		return 0;
> +
> +	for_each_set_bit(bit, &mask, BITS_PER_TYPE(mask)) {
> +		/* For each set bit, see if the following bits are set also */
> +		unsigned long bitfield = GENMASK(bit + width - 1, bit);
> +
> +		if ((mask & bitfield) == bitfield)
> +			out_mask |= BIT(bit);
> +	}
> +
> +	return out_mask;

This is very clever!

This fits very nicely with the part where we want to check if joiner is 
supported "for a specific crtc" during compute_config phase.

This works even for width = 1 as well, but it's better to handle the 
case separately as you have already done, it's intuitive and avoids 
iterating through the bits.

Reviewed-by: Ankit Nautiyal <[email protected]>


> +}
> +
> +/*
> + * Return a bitmask of all valid joiner primary pipes for joining
> + * num_joined_pipes pipes. For completeness, return all valid pipes for
> + * num_joined_pipes == 1.
> + *
> + * Return 0 if the platform doesn't support joining for the requested number of
> + * pipes, or there are not enough consecutive pipes available.
> + */
> +u8 intel_joiner_valid_primary_pipe_mask(struct intel_display *display, int num_joined_pipes)
> +{
> +	if (num_joined_pipes == 1) {
> +		return DISPLAY_RUNTIME_INFO(display)->pipe_mask;
> +	} else if (num_joined_pipes == 2) {
> +		if (!HAS_UNCOMPRESSED_JOINER(display) && !HAS_BIGJOINER(display))
> +			return 0;
> +	} else if (num_joined_pipes == 4) {
> +		if (!HAS_ULTRAJOINER(display))
> +			return 0;
> +	} else {
> +		return 0;
> +	}
> +
> +	return find_consecutive_bits(DISPLAY_RUNTIME_INFO(display)->pipe_mask, num_joined_pipes);
> +}
> +
>   static enum pipe joiner_primary_pipe(const struct intel_crtc_state *crtc_state)
>   {
>   	return ffs(crtc_state->joiner_pipes) - 1;
> @@ -8113,9 +8160,9 @@ static int max_dotclock(struct intel_display *display)
>   {
>   	int max_dotclock = display->cdclk.max_dotclk_freq;
>   
> -	if (HAS_ULTRAJOINER(display))
> +	if (intel_joiner_valid_primary_pipe_mask(display, 4))
>   		max_dotclock *= 4;
> -	else if (HAS_UNCOMPRESSED_JOINER(display) || HAS_BIGJOINER(display))
> +	else if (intel_joiner_valid_primary_pipe_mask(display, 2))
>   		max_dotclock *= 2;
>   
>   	return max_dotclock;
> diff --git a/drivers/gpu/drm/i915/display/intel_display.h b/drivers/gpu/drm/i915/display/intel_display.h
> index 57ea4f2edf2a..e7ecae9d2d21 100644
> --- a/drivers/gpu/drm/i915/display/intel_display.h
> +++ b/drivers/gpu/drm/i915/display/intel_display.h
> @@ -373,6 +373,7 @@ intel_cpu_transcoder_mode_valid(struct intel_display *display,
>   enum phy intel_port_to_phy(struct intel_display *display, enum port port);
>   bool is_trans_port_sync_mode(const struct intel_crtc_state *state);
>   bool is_trans_port_sync_master(const struct intel_crtc_state *state);
> +u8 intel_joiner_valid_primary_pipe_mask(struct intel_display *display, int num_joined_pipes);
>   u8 intel_crtc_joined_pipe_mask(const struct intel_crtc_state *crtc_state);
>   bool intel_crtc_is_joiner_secondary(const struct intel_crtc_state *crtc_state);
>   bool intel_crtc_is_joiner_primary(const struct intel_crtc_state *crtc_state);