Re: [PATCH v3 8/9] drm/vc4: hdmi: Bump BCM2835 max pixel clock to 165 MHz

[Dave Stevenson]

On Wed, 14 Oct 2026 at 02:03, Cristian Ciocaltea
<cristian.ciocaltea@xxxxxxxxxxxxx> wrote:
>
> Historically, the HSM clock for BCM2835 was fixed at 163.7 MHz.  Since
> the pixel clock had to be at least 1% below the HSM clock, it was capped
> at 162 MHz.
>
> Since commit cd4cb49dc5bb ("drm/vc4: hdmi: Adjust HSM clock rate
> depending on pixel rate"), the HSM clock rate is set to pixel clock *
> 101 / 100.  For a 165 MHz pixel clock, this gives an HSM rate of 166.65
> MHz, which, per Dave Stevenson, the hardware can handle without issues.
>
> Increase the max pixel clock to 165 MHz, which matches the maximum TMDS
> character rate of HDMI 1.0 through 1.2.
>
> Suggested-by: Dave Stevenson <dave.stevenson@xxxxxxxxxxxxxxx>
> Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@xxxxxxxxxxxxx>

Reviewed-by: Dave Stevenson <dave.stevenson@xxxxxxxxxxxxxxx>

> ---
>  drivers/gpu/drm/vc4/vc4_hdmi.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/vc4/vc4_hdmi.c b/drivers/gpu/drm/vc4/vc4_hdmi.c
> index 0a95e14792e8..5b7dedd7a9a1 100644
> --- a/drivers/gpu/drm/vc4/vc4_hdmi.c
> +++ b/drivers/gpu/drm/vc4/vc4_hdmi.c
> @@ -1319,7 +1319,7 @@ static void vc4_hdmi_encoder_pre_crtc_configure(struct drm_encoder *encoder,
>          * users are able to use pixel clocks closer to HSM's, namely for
>          * 1920x1200@60Hz. So it was decided to have leave a 1% margin between
>          * both clocks. Which, for RPi0-3 implies a maximum pixel clock of
> -        * 162MHz.
> +        * 165MHz.
>          *
>          * Additionally, the AXI clock needs to be at least 25% of
>          * pixel clock, but HSM ends up being the limiting factor.
> @@ -3190,7 +3190,7 @@ static const struct vc4_hdmi_variant bcm2835_variant = {
>         .encoder_type           = VC4_ENCODER_TYPE_HDMI0,
>         .debugfs_name           = "hdmi_regs",
>         .card_name              = "vc4-hdmi",
> -       .max_pixel_clock        = 162000000,
> +       .max_pixel_clock        = HDMI_1_0_TMDS_CHAR_RATE_MAX_HZ,
>         .registers              = vc4_hdmi_fields,
>         .num_registers          = ARRAY_SIZE(vc4_hdmi_fields),
>
>
> --
> 2.55.0
>