Re: [PATCH v4 62/78] drm/vc4: hdmi: Adjust HSM clock rate depending on pixel rate

[Dave Stevenson <dave.stevenson@raspberrypi.com>]

Hi Maxime

On Wed, 8 Jul 2020 at 18:44, Maxime Ripard <maxime@cerno.tech> wrote:
>
> The HSM clock needs to be setup at around 101% of the pixel rate. This
> was done previously by setting the clock rate to 163.7MHz at probe time and
> only check in mode_valid whether the mode pixel clock was under the pixel
> clock +1% or not.
>
> However, with 4k we need to change that frequency to a higher frequency
> than 163.7MHz, and yet want to have the lowest clock as possible to have a
> decent power saving.
>
> Let's change that logic a bit by setting the clock rate of the HSM clock
> to the pixel rate at encoder_enable time. This would work for the
> BCM2711 that support 4k resolutions and has a clock that can provide it,
> but we still have to take care of a 4k panel plugged on a BCM283x SoCs
> that wouldn't be able to use those modes, so let's define the limit in
> the variant.
>
> Signed-off-by: Maxime Ripard <maxime@cerno.tech>

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

> ---
>  drivers/gpu/drm/vc4/vc4_hdmi.c | 79 ++++++++++++++++-------------------
>  drivers/gpu/drm/vc4/vc4_hdmi.h |  3 +-
>  2 files changed, 41 insertions(+), 41 deletions(-)
>
> diff --git a/drivers/gpu/drm/vc4/vc4_hdmi.c b/drivers/gpu/drm/vc4/vc4_hdmi.c
> index 17797b14cde4..9f30fab744f2 100644
> --- a/drivers/gpu/drm/vc4/vc4_hdmi.c
> +++ b/drivers/gpu/drm/vc4/vc4_hdmi.c
> @@ -53,7 +53,6 @@
>  #include "vc4_hdmi_regs.h"
>  #include "vc4_regs.h"
>
> -#define HSM_CLOCK_FREQ 163682864
>  #define CEC_CLOCK_FREQ 40000
>
>  static int vc4_hdmi_debugfs_regs(struct seq_file *m, void *unused)
> @@ -326,6 +325,7 @@ static void vc4_hdmi_encoder_disable(struct drm_encoder *encoder)
>         HDMI_WRITE(HDMI_VID_CTL,
>                    HDMI_READ(HDMI_VID_CTL) & ~VC4_HD_VID_CTL_ENABLE);
>
> +       clk_disable_unprepare(vc4_hdmi->hsm_clock);
>         clk_disable_unprepare(vc4_hdmi->pixel_clock);
>
>         ret = pm_runtime_put(&vc4_hdmi->pdev->dev);
> @@ -423,6 +423,7 @@ static void vc4_hdmi_encoder_enable(struct drm_encoder *encoder)
>         struct vc4_hdmi *vc4_hdmi = encoder_to_vc4_hdmi(encoder);
>         struct vc4_hdmi_encoder *vc4_encoder = to_vc4_hdmi_encoder(encoder);
>         bool debug_dump_regs = false;
> +       unsigned long pixel_rate, hsm_rate;
>         int ret;
>
>         ret = pm_runtime_get_sync(&vc4_hdmi->pdev->dev);
> @@ -431,9 +432,8 @@ static void vc4_hdmi_encoder_enable(struct drm_encoder *encoder)
>                 return;
>         }
>
> -       ret = clk_set_rate(vc4_hdmi->pixel_clock,
> -                          mode->clock * 1000 *
> -                          ((mode->flags & DRM_MODE_FLAG_DBLCLK) ? 2 : 1));
> +       pixel_rate = mode->clock * 1000 * ((mode->flags & DRM_MODE_FLAG_DBLCLK) ? 2 : 1);
> +       ret = clk_set_rate(vc4_hdmi->pixel_clock, pixel_rate);
>         if (ret) {
>                 DRM_ERROR("Failed to set pixel clock rate: %d\n", ret);
>                 return;
> @@ -445,6 +445,36 @@ static void vc4_hdmi_encoder_enable(struct drm_encoder *encoder)
>                 return;
>         }
>
> +       /*
> +        * As stated in RPi's vc4 firmware "HDMI state machine (HSM) clock must
> +        * be faster than pixel clock, infinitesimally faster, tested in
> +        * simulation. Otherwise, exact value is unimportant for HDMI
> +        * operation." This conflicts with bcm2835's vc4 documentation, which
> +        * states HSM's clock has to be at least 108% of the pixel clock.
> +        *
> +        * Real life tests reveal that vc4's firmware statement holds up, and
> +        * 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.
> +        *
> +        * Additionally, the AXI clock needs to be at least 25% of
> +        * pixel clock, but HSM ends up being the limiting factor.
> +        */
> +       hsm_rate = max_t(unsigned long, 120000000, (pixel_rate / 100) * 101);
> +       ret = clk_set_rate(vc4_hdmi->hsm_clock, hsm_rate);
> +       if (ret) {
> +               DRM_ERROR("Failed to set HSM clock rate: %d\n", ret);
> +               return;
> +       }
> +
> +       ret = clk_prepare_enable(vc4_hdmi->hsm_clock);
> +       if (ret) {
> +               DRM_ERROR("Failed to turn on HSM clock: %d\n", ret);
> +               clk_disable_unprepare(vc4_hdmi->pixel_clock);
> +               return;
> +       }
> +
>         if (vc4_hdmi->variant->reset)
>                 vc4_hdmi->variant->reset(vc4_hdmi);
>
> @@ -559,23 +589,9 @@ static enum drm_mode_status
>  vc4_hdmi_encoder_mode_valid(struct drm_encoder *encoder,
>                             const struct drm_display_mode *mode)
>  {
> -       /*
> -        * As stated in RPi's vc4 firmware "HDMI state machine (HSM) clock must
> -        * be faster than pixel clock, infinitesimally faster, tested in
> -        * simulation. Otherwise, exact value is unimportant for HDMI
> -        * operation." This conflicts with bcm2835's vc4 documentation, which
> -        * states HSM's clock has to be at least 108% of the pixel clock.
> -        *
> -        * Real life tests reveal that vc4's firmware statement holds up, and
> -        * 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.
> -        *
> -        * Additionally, the AXI clock needs to be at least 25% of
> -        * pixel clock, but HSM ends up being the limiting factor.
> -        */
> -       if (mode->clock > HSM_CLOCK_FREQ / (1000 * 101 / 100))
> +       struct vc4_hdmi *vc4_hdmi = encoder_to_vc4_hdmi(encoder);
> +
> +       if ((mode->clock * 1000) > vc4_hdmi->variant->max_pixel_clock)
>                 return MODE_CLOCK_HIGH;
>
>         return MODE_OK;
> @@ -1349,23 +1365,6 @@ static int vc4_hdmi_bind(struct device *dev, struct device *master, void *data)
>                 return -EPROBE_DEFER;
>         }
>
> -       /* This is the rate that is set by the firmware.  The number
> -        * needs to be a bit higher than the pixel clock rate
> -        * (generally 148.5Mhz).
> -        */
> -       ret = clk_set_rate(vc4_hdmi->hsm_clock, HSM_CLOCK_FREQ);
> -       if (ret) {
> -               DRM_ERROR("Failed to set HSM clock rate: %d\n", ret);
> -               goto err_put_i2c;
> -       }
> -
> -       ret = clk_prepare_enable(vc4_hdmi->hsm_clock);
> -       if (ret) {
> -               DRM_ERROR("Failed to turn on HDMI state machine clock: %d\n",
> -                         ret);
> -               goto err_put_i2c;
> -       }
> -
>         /* Only use the GPIO HPD pin if present in the DT, otherwise
>          * we'll use the HDMI core's register.
>          */
> @@ -1413,9 +1412,7 @@ static int vc4_hdmi_bind(struct device *dev, struct device *master, void *data)
>  err_destroy_encoder:
>         drm_encoder_cleanup(encoder);
>  err_unprepare_hsm:
> -       clk_disable_unprepare(vc4_hdmi->hsm_clock);
>         pm_runtime_disable(dev);
> -err_put_i2c:
>         put_device(&vc4_hdmi->ddc->dev);
>
>         return ret;
> @@ -1454,7 +1451,6 @@ static void vc4_hdmi_unbind(struct device *dev, struct device *master,
>         vc4_hdmi_connector_destroy(&vc4_hdmi->connector);
>         drm_encoder_cleanup(&vc4_hdmi->encoder.base.base);
>
> -       clk_disable_unprepare(vc4_hdmi->hsm_clock);
>         pm_runtime_disable(dev);
>
>         put_device(&vc4_hdmi->ddc->dev);
> @@ -1479,6 +1475,7 @@ static int vc4_hdmi_dev_remove(struct platform_device *pdev)
>  static const struct vc4_hdmi_variant bcm2835_variant = {
>         .encoder_type           = VC4_ENCODER_TYPE_HDMI0,
>         .debugfs_name           = "hdmi_regs",
> +       .max_pixel_clock        = 162000000,
>         .cec_available          = true,
>         .registers              = vc4_hdmi_fields,
>         .num_registers          = ARRAY_SIZE(vc4_hdmi_fields),
> diff --git a/drivers/gpu/drm/vc4/vc4_hdmi.h b/drivers/gpu/drm/vc4/vc4_hdmi.h
> index 3f07aebe89f1..342f6e0227a2 100644
> --- a/drivers/gpu/drm/vc4/vc4_hdmi.h
> +++ b/drivers/gpu/drm/vc4/vc4_hdmi.h
> @@ -36,6 +36,9 @@ struct vc4_hdmi_variant {
>         /* Set to true when the CEC support is available */
>         bool cec_available;
>
> +       /* Maximum pixel clock supported by the controller (in Hz) */
> +       unsigned long long max_pixel_clock;
> +
>         /* List of the registers available on that variant */
>         const struct vc4_hdmi_register *registers;
>
> --
> git-series 0.9.1