Re: [PATCH 1/2] drm/vc4: hdmi: Disable Wifi Frequencies

[Daniel Vetter <daniel@ffwll.ch>]

On Mon, Sep 28, 2020 at 9:06 AM Maxime Ripard <maxime@cerno.tech> wrote:
>
> There's cross-talk on the RPi4 between the 2.4GHz channels used by the WiFi
> chip and some resolutions, most notably 1440p at 60Hz.
>
> In such a case, we can either reject entirely the mode, or lower slightly
> the pixel frequency to remove the overlap. Let's go for the latter.
>
> Signed-off-by: Maxime Ripard <maxime@cerno.tech>
> ---
>  .../bindings/display/brcm,bcm2711-hdmi.yaml        |  6 ++++++
>  drivers/gpu/drm/vc4/vc4_hdmi.c                     | 14 +++++++++++++-
>  drivers/gpu/drm/vc4/vc4_hdmi.h                     |  8 ++++++++
>  3 files changed, 27 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/devicetree/bindings/display/brcm,bcm2711-hdmi.yaml b/Documentation/devicetree/bindings/display/brcm,bcm2711-hdmi.yaml
> index 03a76729d26c..63e7fe999c0a 100644
> --- a/Documentation/devicetree/bindings/display/brcm,bcm2711-hdmi.yaml
> +++ b/Documentation/devicetree/bindings/display/brcm,bcm2711-hdmi.yaml
> @@ -76,6 +76,12 @@ properties:
>    resets:
>      maxItems: 1
>
> +  raspberrypi,disable-wifi-frequencies:
> +    type: boolean
> +    description: >
> +      Should the pixel frequencies in the WiFi frequencies range be
> +      avoided?
> +
>  required:
>    - compatible
>    - reg
> diff --git a/drivers/gpu/drm/vc4/vc4_hdmi.c b/drivers/gpu/drm/vc4/vc4_hdmi.c
> index acfb4e235214..74da7c00ecd0 100644
> --- a/drivers/gpu/drm/vc4/vc4_hdmi.c
> +++ b/drivers/gpu/drm/vc4/vc4_hdmi.c
> @@ -877,13 +877,22 @@ static int vc4_hdmi_encoder_atomic_check(struct drm_encoder *encoder,
>         struct vc4_hdmi_connector_state *vc4_state = conn_state_to_vc4_hdmi_conn_state(conn_state);
>         struct drm_display_mode *mode = &crtc_state->adjusted_mode;
>         struct vc4_hdmi *vc4_hdmi = encoder_to_vc4_hdmi(encoder);
> -       unsigned long long pixel_rate = mode->clock * 1000;
> +       unsigned long long pixel_rate;
>
>         if (vc4_hdmi->variant->broken_odd_h_timings &&
>             ((mode->hdisplay % 2) || (mode->hsync_start % 2) ||
>              (mode->hsync_end % 2) || (mode->htotal % 2)))
>                 return -EINVAL;
>
> +       /*
> +        * The 1440p@60 pixel rate is in the same range than the WiFi
> +        * channels. Slightly lower the frequency to bring it out of the
> +        * WiFi range.
> +        */
> +       if (vc4_hdmi->disable_wifi_frequencies && mode->clock == 241500)
> +               mode->clock = 238560;

Don't you want to map for a (narrow) range of frequencies here? Just
for that infamous 60p vs 59.99p thing and similar. And I think that
would still be in that band you want to avoid.
-Daniel

> +
> +       pixel_rate = mode->clock * 1000;
>         if (mode->flags & DRM_MODE_FLAG_DBLCLK)
>                 pixel_rate *= 2;
>
> @@ -1837,6 +1846,9 @@ static int vc4_hdmi_bind(struct device *dev, struct device *master, void *data)
>                 vc4_hdmi->hpd_active_low = hpd_gpio_flags & OF_GPIO_ACTIVE_LOW;
>         }
>
> +       vc4_hdmi->disable_wifi_frequencies =
> +               of_property_read_bool(dev->of_node, "raspberrypi,disable-wifi-frequencies");
> +
>         pm_runtime_enable(dev);
>
>         drm_simple_encoder_init(drm, encoder, DRM_MODE_ENCODER_TMDS);
> diff --git a/drivers/gpu/drm/vc4/vc4_hdmi.h b/drivers/gpu/drm/vc4/vc4_hdmi.h
> index 40e51ece8efe..6618ee758890 100644
> --- a/drivers/gpu/drm/vc4/vc4_hdmi.h
> +++ b/drivers/gpu/drm/vc4/vc4_hdmi.h
> @@ -143,6 +143,14 @@ struct vc4_hdmi {
>         int hpd_gpio;
>         bool hpd_active_low;
>
> +       /*
> +        * On some systems (like the RPi4), some modes are in the same
> +        * frequency range than the WiFi channels (1440p@60Hz for
> +        * example). Should we take evasive actions because that system
> +        * has a wifi adapter.
> +        */
> +       bool disable_wifi_frequencies;
> +
>         struct cec_adapter *cec_adap;
>         struct cec_msg cec_rx_msg;
>         bool cec_tx_ok;
> --
> 2.26.2
>
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch