[PATCH 3/8] drm/vc4: hdmi: Properly compute the BVB clock rate

[Dave Stevenson]

On Thu, 25 Feb 2021 at 16:44, Dave Stevenson
<dave.stevenson at raspberrypi.com> wrote:
>
> Hi Maxime
>
> On Thu, 25 Feb 2021 at 15:59, Maxime Ripard <maxime at cerno.tech> wrote:
> >
> > The BVB clock rate computation doesn't take into account a mode clock of
> > 594MHz that we're going to need to support 4k60.
> >
> > Signed-off-by: Maxime Ripard <maxime at cerno.tech>
> > ---
> >  drivers/gpu/drm/vc4/vc4_hdmi.c | 11 +++--------
> >  1 file changed, 3 insertions(+), 8 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/vc4/vc4_hdmi.c b/drivers/gpu/drm/vc4/vc4_hdmi.c
> > index eee9751009c2..b5bc742993a4 100644
> > --- a/drivers/gpu/drm/vc4/vc4_hdmi.c
> > +++ b/drivers/gpu/drm/vc4/vc4_hdmi.c
> > @@ -91,7 +91,6 @@
> >  # define VC4_HD_M_ENABLE                       BIT(0)
> >
> >  #define CEC_CLOCK_FREQ 40000
> > -#define VC4_HSM_MID_CLOCK 149985000
> >
> >  #define HDMI_14_MAX_TMDS_CLK   (340 * 1000 * 1000)
> >
> > @@ -739,7 +738,7 @@ static void vc4_hdmi_encoder_pre_crtc_configure(struct drm_encoder *encoder,
> >                 conn_state_to_vc4_hdmi_conn_state(conn_state);
> >         struct drm_display_mode *mode = &encoder->crtc->state->adjusted_mode;
> >         struct vc4_hdmi *vc4_hdmi = encoder_to_vc4_hdmi(encoder);
> > -       unsigned long pixel_rate, hsm_rate;
> > +       unsigned long bvb_rate, pixel_rate, hsm_rate;
> >         int ret;
> >
> >         ret = pm_runtime_get_sync(&vc4_hdmi->pdev->dev);
> > @@ -793,12 +792,8 @@ static void vc4_hdmi_encoder_pre_crtc_configure(struct drm_encoder *encoder,
> >
> >         vc4_hdmi_cec_update_clk_div(vc4_hdmi);
> >
> > -       /*
> > -        * FIXME: When the pixel freq is 594MHz (4k60), this needs to be setup
> > -        * at 300MHz.
> > -        */
> > -       ret = clk_set_min_rate(vc4_hdmi->pixel_bvb_clock,
> > -                              (hsm_rate > VC4_HSM_MID_CLOCK ? 150000000 : 75000000));
> > +       bvb_rate = roundup(mode->clock * 1000 / 2, 75000000);
>
> Minor hesitation here that I would need to check the hardware over.
> As I read your code, if the clock falls 300MHz and 450MHz then you'd
> ask for a bvb_rate of 225MHz. Depending on what the source clock is
> that may not be valid.
> The firmware goes for 75, 150, or 300MHz only.

The information I have says it has to be an integer divide of 600MHz
(PLLC @ 3GHz / 5), so 225MHz is not valid.

(Minor contradictory information though as PLLC is bumped to 3.3GHz
with enable_4kp60, but we still appear to get 300MHz for BVB after the
/5. A little more checking warranted around here).

>   Dave
>
> > +       ret = clk_set_min_rate(vc4_hdmi->pixel_bvb_clock, bvb_rate);
> >         if (ret) {
> >                 DRM_ERROR("Failed to set pixel bvb clock rate: %d\n", ret);
> >                 clk_disable_unprepare(vc4_hdmi->hsm_clock);
> > --
> > 2.29.2
> >