linux-kernel - Re: [PATCH v1 2/5] drm/mediatek: Fix kobject put for component sub-drivers

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Message-ID: <CAGXv+5GsoHN5mVe_kEP+oAzxa9HqgfhoiDqvO3xiQurFrvQ3qA@mail.gmail.com>
Date: Wed, 2 Apr 2025 18:18:41 +0800
From: Chen-Yu Tsai <wenst@...omium.org>
To: AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>
Cc: chunkuang.hu@...nel.org, p.zabel@...gutronix.de, airlied@...il.com, 
	simona@...ll.ch, matthias.bgg@...il.com, nancy.lin@...iatek.com, 
	ck.hu@...iatek.com, djkurtz@...omium.org, littlecvr@...omium.org, 
	bibby.hsieh@...iatek.com, dri-devel@...ts.freedesktop.org, 
	linux-mediatek@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, kernel@...labora.com
Subject: Re: [PATCH v1 2/5] drm/mediatek: Fix kobject put for component sub-drivers

On Wed, Apr 2, 2025 at 4:36 PM AngeloGioacchino Del Regno
<angelogioacchino.delregno@...labora.com> wrote:
>
> In function mtk_drm_get_all_priv(), this driver is incrementing

              ^ mtk_drm_get_all_drm_priv()

> the refcount for the sub-drivers of mediatek-drm with a call to
> device_find_child() when taking a reference to all of those child
> devices.

Looking closer, that function is also missing a platform_device_put()
to match of_find_device_by_node().

> When the component bind fails multiple times this results in a
> refcount_t overflow, as the reference count is never decremented:
> fix that by adding a call to put_device() for all of the mmsys
> devices in a loop, in error cases of mtk_drm_bind() and in the
> mtk_drm_unbind() callback.
>
> Fixes: 1ef7ed48356c ("drm/mediatek: Modify mediatek-drm for mt8195 multi mmsys support")
> Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>

Reviewed-by: Chen-Yu Tsai <wenst@...omium.org>

> ---
>  drivers/gpu/drm/mediatek/mtk_drm_drv.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/drivers/gpu/drm/mediatek/mtk_drm_drv.c b/drivers/gpu/drm/mediatek/mtk_drm_drv.c
> index a8fbccb50c74..6b31df587507 100644
> --- a/drivers/gpu/drm/mediatek/mtk_drm_drv.c
> +++ b/drivers/gpu/drm/mediatek/mtk_drm_drv.c
> @@ -676,6 +676,10 @@ static int mtk_drm_bind(struct device *dev)
>         for (i = 0; i < private->data->mmsys_dev_num; i++)
>                 private->all_drm_private[i]->drm = NULL;
>  err_put_dev:
> +       for (i = 0; i < private->data->mmsys_dev_num; i++) {
> +               /* For device_find_child in mtk_drm_get_all_priv() */
> +               put_device(private->all_drm_private[i]->dev);
> +       }
>         put_device(private->mutex_dev);
>         return ret;
>  }
> @@ -683,6 +687,7 @@ static int mtk_drm_bind(struct device *dev)
>  static void mtk_drm_unbind(struct device *dev)
>  {
>         struct mtk_drm_private *private = dev_get_drvdata(dev);
> +       int i;
>
>         /* for multi mmsys dev, unregister drm dev in mmsys master */
>         if (private->drm_master) {
> @@ -690,6 +695,10 @@ static void mtk_drm_unbind(struct device *dev)
>                 mtk_drm_kms_deinit(private->drm);
>                 drm_dev_put(private->drm);
>
> +               for (i = 0; i < private->data->mmsys_dev_num; i++) {

Just FYI you can also declare the int within the for statement.

ChenYu

> +                       /* For device_find_child in mtk_drm_get_all_priv() */
> +                       put_device(private->all_drm_private[i]->dev);
> +               }
>                 put_device(private->mutex_dev);
>         }
>         private->mtk_drm_bound = false;
> --
> 2.48.1
>