Re: [PATCH 1/8] drm/display: hdmi: Add common TMDS character rate constants

[Javier Martinez Canillas]

Jani Nikula <jani.nikula@xxxxxxxxx> writes:

Hello Jani,

> On Tue, 19 May 2026, Javier Martinez Canillas <javierm@xxxxxxxxxx> wrote:

[...]

>>  
>> +/* HDMI spec TMDS character rate limits (in Hz) */
>> +#define DRM_HDMI_TMDS_CHAR_RATE_MIN	25000000
>> +#define DRM_HDMI_TMDS_CHAR_RATE_MAX_1_0	165000000
>> +#define DRM_HDMI_TMDS_CHAR_RATE_MAX_1_3	340000000
>> +#define DRM_HDMI_TMDS_CHAR_RATE_MAX_2_0	600000000
>
> Usually everything in DRM is in kHz, and Hz is the exception.
>

That is correct but in this case these constants are to be used with the
HDMI helpers. Both struct drm_connector_hdmi_state.tmds_char_rate and
the struct drm_bridge_funcs.hdmi_tmds_char_rate_valid() callback expect
the TMDS char rate to be defined in Hz.

If we define these in kHz, it means that drivers will have to * 1000 at
every call site.

> I'm also not sure the 1_0, 1_3, and 2_0 really help anyone reading the
> code. I won't remember what they mean in Hz or kHz, and I'll have to
> look them up every single time.
>

I discussed this with Maxime before posting the patches since I wondered
the same. He suggested that the max TMDS character rate was linked to the
HDMI spec versions and that it would be more readable to name it using
the spec version rather than the resolution.

The other naming I suggested was:

/* HDMI spec TMDS character rate limits (in Hz) */
#define DRM_HDMI_TMDS_CHAR_RATE_MIN		 25000000
#define DRM_HDMI_TMDS_CHAR_RATE_74_25MHZ	 74250000
#define DRM_HDMI_TMDS_CHAR_RATE_148_5MHZ	148500000
#define DRM_HDMI_TMDS_CHAR_RATE_297MHZ		297000000
#define DRM_HDMI_TMDS_CHAR_RATE_MAX_1_4		340000000

> BR,
> Jani.
>

-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat