Re: [PATCH v7 4/6] iommu/arm-smmu: Add the device_link between masters and smmu

[Tomasz Figa]

Hi Vivek,

Thanks for the patch. Please see my comments inline.

On Wed, Feb 7, 2018 at 7:31 PM, Vivek Gautam
<vivek.gautam@xxxxxxxxxxxxxx> wrote:
> From: Sricharan R <sricharan@xxxxxxxxxxxxxx>
>
> Finally add the device link between the master device and
> smmu, so that the smmu gets runtime enabled/disabled only when the
> master needs it. This is done from add_device callback which gets
> called once when the master is added to the smmu.
>
> Signed-off-by: Sricharan R <sricharan@xxxxxxxxxxxxxx>
> Signed-off-by: Vivek Gautam <vivek.gautam@xxxxxxxxxxxxxx>
> ---
>  drivers/iommu/arm-smmu.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
>
> diff --git a/drivers/iommu/arm-smmu.c b/drivers/iommu/arm-smmu.c
> index c024f69c1682..c7e924d553bd 100644
> --- a/drivers/iommu/arm-smmu.c
> +++ b/drivers/iommu/arm-smmu.c
> @@ -215,6 +215,9 @@ struct arm_smmu_device {
>
>         /* IOMMU core code handle */
>         struct iommu_device             iommu;
> +
> +       /* runtime PM link to master */
> +       struct device_link *link;
>  };
>
>  enum arm_smmu_context_fmt {
> @@ -1425,6 +1428,17 @@ static int arm_smmu_add_device(struct device *dev)
>
>         pm_runtime_put_sync(smmu->dev);
>
> +       /*
> +        * Establish the link between smmu and master, so that the
> +        * smmu gets runtime enabled/disabled as per the master's
> +        * needs.
> +        */
> +       smmu->link = device_link_add(dev, smmu->dev, DL_FLAG_PM_RUNTIME);
> +       if (!smmu->link)
> +               dev_warn(smmu->dev,
> +                        "Unable to create device link between %s and %s\n",
> +                        dev_name(smmu->dev), dev_name(dev));

How likely it is that the master can work normally even if the link
add fails? Perhaps we should just return an error here?

> +
>         return 0;
>
>  out_rpm_put:
> @@ -1449,6 +1463,8 @@ static void arm_smmu_remove_device(struct device *dev)
>         cfg  = fwspec->iommu_priv;
>         smmu = cfg->smmu;
>
> +       device_link_del(smmu->link);

We allowed smmu->link in arm_smmu_add_device(), but here we don't
check it. Looking at the code, device_link_del() doesn't seem to check
either.

Note that this problem would go away if we fail add_device on
device_link_add() failure, as I suggested above, so no change would be
necessary.

Best regards,
Tomasz