Re: [PATCH 2/2] drm/display: hdmi: Round odd max_bpc down to even numbers

[Maxime Ripard]

On Mon, Jun 08, 2026 at 01:19:08PM +0200, Nicolas Frattaroli wrote:
> The HDMI state helpers will count down from the max bpc to 8 in steps of
> 2, trying each value as a possible output bpc. This goes awry if max bpc
> is restricted by userspace to an odd number with the "max bpc" connector
> property.
> 
> Prevent this, without introducing any additional bpc format trial steps,
> by simply subtracting max_bpc modulo 2 from max_bpc as the starting
> point for the for loop.
> 
> Additionally, add a KUnit test to validate the handling of this.
> 
> Fixes: 26ff1c38fc29 ("drm/connector: hdmi: Compute bpc and format automatically")
> Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@xxxxxxxxxxxxx>
> ---
>  drivers/gpu/drm/display/drm_hdmi_state_helper.c    |  2 +-
>  drivers/gpu/drm/tests/drm_hdmi_state_helper_test.c | 71 ++++++++++++++++++++++
>  2 files changed, 72 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/display/drm_hdmi_state_helper.c b/drivers/gpu/drm/display/drm_hdmi_state_helper.c
> index 8303475ec021..9fbf88054ad8 100644
> --- a/drivers/gpu/drm/display/drm_hdmi_state_helper.c
> +++ b/drivers/gpu/drm/display/drm_hdmi_state_helper.c
> @@ -648,7 +648,7 @@ hdmi_compute_format_bpc(const struct drm_connector *connector,
>  	unsigned int bpc;
>  	int ret;
>  
> -	for (bpc = max_bpc; bpc >= 8; bpc -= 2) {
> +	for (bpc = max_bpc - max_bpc % 2; bpc >= 8; bpc -= 2) {

bpc = round_down(max_bpc, 2) (or rounddown) would be more readable here.

Looks good otherwise, once fixed
Reviewed-by: Maxime Ripard <mripard@xxxxxxxxxx>

Maxime

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