linux-kernel - Re: [PATCH v4 2/3] iommu/tegra-smmu: Rework tegra_smmu_probe

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Date:   Mon, 5 Oct 2020 18:05:46 -0700
From:   Nicolin Chen <nicoleotsuka@...il.com>
To:     Thierry Reding <thierry.reding@...il.com>
Cc:     Dmitry Osipenko <digetx@...il.com>, joro@...tes.org,
        vdumpa@...dia.com, jonathanh@...dia.com,
        linux-tegra@...r.kernel.org, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/3] iommu/tegra-smmu: Rework tegra_smmu_probe_device()

On Mon, Oct 05, 2020 at 11:57:54AM +0200, Thierry Reding wrote:
> On Fri, Oct 02, 2020 at 11:58:29AM -0700, Nicolin Chen wrote:
> > On Fri, Oct 02, 2020 at 06:02:18PM +0300, Dmitry Osipenko wrote:
> > > 02.10.2020 09:08, Nicolin Chen пишет:
> > > >  static int tegra_smmu_of_xlate(struct device *dev,
> > > >  			       struct of_phandle_args *args)
> > > >  {
> > > > +	struct platform_device *iommu_pdev = of_find_device_by_node(args->np);
> > > > +	struct tegra_mc *mc = platform_get_drvdata(iommu_pdev);
> > > >  	u32 id = args->args[0];
> > > >  
> > > > +	of_node_put(args->np);
> > > 
> > > of_find_device_by_node() takes device reference and not the np
> > > reference. This is a bug, please remove of_node_put().
> > 
> > Looks like so. Replacing it with put_device(&iommu_pdev->dev);
> 
> Putting the put_device() here is wrong, though. You need to make sure
> you keep a reference to it as long as you keep accessing the data that
> is owned by it.

I am confused. You said in the other reply (to Dmitry) that we do
need to put_device(mc->dev), where mc->dev should be the same as
iommu_pdev->dev. But here your comments sounds that we should not
put_device at all since ->probe_device/group_device/attach_dev()
will use it later.

> Like I said earlier, this is a bit weird in this case because we're
> self-referencing, so iommu_pdev->dev is going to stay around as long as
> the SMMU is. However, it might be worth to properly track the lifetime
> anyway just so that the code can serve as a good example of how to do
> things.

What's this "track-the-lifetime"?

> If you decide to go for the shortcut and not track this reference
> properly, then at least you need to add a comment as to why it is safe
> to do in this case. This ensures that readers are away of the
> circumstances and don't copy this bad code into a context where the
> circumstances are different.

I don't quite get this "shortcut" here either...mind elaborating?