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From:   Dave Stevenson <dave.stevenson@raspberrypi.com>
Date:   Tue, 28 Jul 2020 13:32:22 +0100
Message-ID: <CAPY8ntAWq_3=K_=zHVRmBqgA-ZKMbjKwrWvg+Y6FtSjBjc_DvA@mail.gmail.com>
Subject: Re: [PATCH v4 47/78] drm/vc4: hdmi: Retrieve the vc4_hdmi at unbind
 using our device
To:     Maxime Ripard <maxime@cerno.tech>
Cc:     Nicolas Saenz Julienne <nsaenzjulienne@suse.de>,
        Eric Anholt <eric@anholt.net>,
        DRI Development <dri-devel@lists.freedesktop.org>,
        linux-rpi-kernel@lists.infradead.org,
        bcm-kernel-feedback-list@broadcom.com,
        linux-arm-kernel@lists.infradead.org,
        LKML <linux-kernel@vger.kernel.org>,
        Tim Gover <tim.gover@raspberrypi.com>,
        Phil Elwell <phil@raspberrypi.com>
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Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Hi Maxime

On Wed, 8 Jul 2020 at 18:43, Maxime Ripard <maxime@cerno.tech> wrote:
>
> The unbind function needs to retrieve a vc4_hdmi structure pointer through
> the struct device that we're given since we want to support multiple HDMI
> controllers.
>
> However, our optional ASoC support doesn't make that trivial since it will
> overwrite the device drvdata if we use it, but obviously won't if we don't
> use it.
>
> Let's make sure the fields are at the proper offset to be able to cast
> between the snd_soc_card structure and the vc4_hdmi structure
> transparently so we can support both cases.
>
> Signed-off-by: Maxime Ripard <maxime@cerno.tech>

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

> ---
>  drivers/gpu/drm/vc4/vc4_hdmi.c | 24 +++++++++++++++++++++++-
>  drivers/gpu/drm/vc4/vc4_hdmi.h |  4 ++--
>  2 files changed, 25 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/vc4/vc4_hdmi.c b/drivers/gpu/drm/vc4/vc4_hdmi.c
> index 09b297a1b39d..7cd1394c10fa 100644
> --- a/drivers/gpu/drm/vc4/vc4_hdmi.c
> +++ b/drivers/gpu/drm/vc4/vc4_hdmi.c
> @@ -1200,6 +1200,7 @@ static int vc4_hdmi_bind(struct device *dev, struct device *master, void *data)
>         if (!vc4_hdmi)
>                 return -ENOMEM;
>
> +       dev_set_drvdata(dev, vc4_hdmi);
>         encoder = &vc4_hdmi->encoder.base.base;
>         vc4_hdmi->encoder.base.type = VC4_ENCODER_TYPE_HDMI0;
>         vc4_hdmi->pdev = pdev;
> @@ -1362,7 +1363,28 @@ static void vc4_hdmi_unbind(struct device *dev, struct device *master,
>  {
>         struct drm_device *drm = dev_get_drvdata(master);
>         struct vc4_dev *vc4 = drm->dev_private;
> -       struct vc4_hdmi *vc4_hdmi = vc4->hdmi;
> +       struct vc4_hdmi *vc4_hdmi;
> +
> +       /*
> +        * ASoC makes it a bit hard to retrieve a pointer to the
> +        * vc4_hdmi structure. Registering the card will overwrite our
> +        * device drvdata with a pointer to the snd_soc_card structure,
> +        * which can then be used to retrieve whatever drvdata we want
> +        * to associate.
> +        *
> +        * However, that doesn't fly in the case where we wouldn't
> +        * register an ASoC card (because of an old DT that is missing
> +        * the dmas properties for example), then the card isn't
> +        * registered and the device drvdata wouldn't be set.
> +        *
> +        * We can deal with both cases by making sure a snd_soc_card
> +        * pointer and a vc4_hdmi structure are pointing to the same
> +        * memory address, so we can treat them indistinctly without any
> +        * issue.
> +        */
> +       BUILD_BUG_ON(offsetof(struct vc4_hdmi_audio, card) != 0);
> +       BUILD_BUG_ON(offsetof(struct vc4_hdmi, audio) != 0);
> +       vc4_hdmi = dev_get_drvdata(dev);
>
>         cec_unregister_adapter(vc4_hdmi->cec_adap);
>         vc4_hdmi_connector_destroy(&vc4_hdmi->connector.base);
> diff --git a/drivers/gpu/drm/vc4/vc4_hdmi.h b/drivers/gpu/drm/vc4/vc4_hdmi.h
> index 749a807cd1f3..d43462789450 100644
> --- a/drivers/gpu/drm/vc4/vc4_hdmi.h
> +++ b/drivers/gpu/drm/vc4/vc4_hdmi.h
> @@ -53,13 +53,13 @@ struct vc4_hdmi_audio {
>
>  /* General HDMI hardware state. */
>  struct vc4_hdmi {
> +       struct vc4_hdmi_audio audio;
> +
>         struct platform_device *pdev;
>
>         struct vc4_hdmi_encoder encoder;
>         struct vc4_hdmi_connector connector;
>
> -       struct vc4_hdmi_audio audio;
> -
>         struct i2c_adapter *ddc;
>         void __iomem *hdmicore_regs;
>         void __iomem *hd_regs;
> --
> git-series 0.9.1