linux-kernel - Re: [PATCH 06/15] drm/vc4: hdmi: Compute the CEC clock divider from the clock rate

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Date:   Fri, 18 Dec 2020 11:30:27 +0000
From:   Dave Stevenson <dave.stevenson@...pberrypi.com>
To:     Maxime Ripard <maxime@...no.tech>
Cc:     Eric Anholt <eric@...olt.net>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Daniel Vetter <daniel.vetter@...el.com>,
        David Airlie <airlied@...ux.ie>,
        Jason Cooper <jason@...edaemon.net>,
        bcm-kernel-feedback-list@...adcom.com,
        linux-arm-kernel@...ts.infradead.org,
        Marc Zyngier <maz@...nel.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Hans Verkuil <hverkuil-cisco@...all.nl>,
        LKML <linux-kernel@...r.kernel.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-rpi-kernel@...ts.infradead.org,
        DRI Development <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH 06/15] drm/vc4: hdmi: Compute the CEC clock divider from
 the clock rate

Hi Maxime

On Thu, 10 Dec 2020 at 13:47, Maxime Ripard <maxime@...no.tech> wrote:
>
> The CEC clock divider needs to output a frequency of 40kHz from the HSM
> rate on the BCM2835. The driver used to have a fixed frequency for it,
> but that changed and we now need to compute it dynamically to maintain
> the proper rate.
>
> Fixes: cd4cb49dc5bb ("drm/vc4: hdmi: Adjust HSM clock rate depending on pixel rate")
> Signed-off-by: Maxime Ripard <maxime@...no.tech>

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

(To be a total pedant it's still a fixed frequency on vc4, but it's
configurable via the variant entry).

> ---
>  drivers/gpu/drm/vc4/vc4_hdmi.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/vc4/vc4_hdmi.c b/drivers/gpu/drm/vc4/vc4_hdmi.c
> index eff3bac562c6..0c53d7427d15 100644
> --- a/drivers/gpu/drm/vc4/vc4_hdmi.c
> +++ b/drivers/gpu/drm/vc4/vc4_hdmi.c
> @@ -1586,6 +1586,7 @@ static int vc4_hdmi_cec_init(struct vc4_hdmi *vc4_hdmi)
>  {
>         struct cec_connector_info conn_info;
>         struct platform_device *pdev = vc4_hdmi->pdev;
> +       u16 clk_cnt;
>         u32 value;
>         int ret;
>
> @@ -1611,8 +1612,9 @@ static int vc4_hdmi_cec_init(struct vc4_hdmi *vc4_hdmi)
>          * divider: the hsm_clock rate and this divider setting will
>          * give a 40 kHz CEC clock.
>          */
> +       clk_cnt = clk_get_rate(vc4_hdmi->hsm_clock) / CEC_CLOCK_FREQ;
>         value |= VC4_HDMI_CEC_ADDR_MASK |
> -                (4091 << VC4_HDMI_CEC_DIV_CLK_CNT_SHIFT);
> +                (clk_cnt << VC4_HDMI_CEC_DIV_CLK_CNT_SHIFT);
>         HDMI_WRITE(HDMI_CEC_CNTRL_1, value);
>         ret = devm_request_threaded_irq(&pdev->dev, platform_get_irq(pdev, 0),
>                                         vc4_cec_irq_handler,
> --
> 2.28.0
>