linux-kernel - Re: [PATCH v3 2/3] iommu/tegra-smmu: Rework .probe_device and .attach

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Date:   Thu, 1 Oct 2020 11:51:52 +0200
From:   Thierry Reding <thierry.reding@...il.com>
To:     Nicolin Chen <nicoleotsuka@...il.com>
Cc:     Dmitry Osipenko <digetx@...il.com>, joro@...tes.org,
        krzk@...nel.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 v3 2/3] iommu/tegra-smmu: Rework .probe_device and
 .attach_dev

On Wed, Sep 30, 2020 at 06:26:30PM -0700, Nicolin Chen wrote:
> On Thu, Oct 01, 2020 at 12:56:46AM +0300, Dmitry Osipenko wrote:
> > 01.10.2020 00:32, Nicolin Chen пишет:
> > > On Thu, Oct 01, 2020 at 12:24:25AM +0300, Dmitry Osipenko wrote:
> > >> ...
> > >>>> It looks to me like the only reason why you need this new global API is
> > >>>> because PCI devices may not have a device tree node with a phandle to
> > >>>> the IOMMU. However, SMMU support for PCI will only be enabled if the
> > >>>> root complex has an iommus property, right? In that case, can't we
> > >>>> simply do something like this:
> > >>>>
> > >>>> 	if (dev_is_pci(dev))
> > >>>> 		np = find_host_bridge(dev)->of_node;
> > >>>> 	else
> > >>>> 		np = dev->of_node;
> > >>>>
> > >>>> ? I'm not sure exactly what find_host_bridge() is called, but I'm pretty
> > >>>> sure that exists.
> > >>>>
> > >>>> Once we have that we can still iterate over the iommus property and do
> > >>>> not need to rely on this global variable.
> > >>>
> > >>> I agree that it'd work. But I was hoping to simplify the code
> > >>> here if it's possible. Looks like we have an argument on this
> > >>> so I will choose to go with your suggestion above for now.
> > >>
> > >> This patch removed more lines than were added. If this will be opposite
> > >> for the Thierry's suggestion, then it's probably not a great suggestion.
> > > 
> > > Sorry, I don't quite understand this comments. Would you please
> > > elaborate what's this "it" being "not a great suggestion"?
> > > 
> > 
> > I meant that you should try to implement Thierry's solution, but if the
> > end result will be worse than the current patch, then you shouldn't make
> > a v4, but get back to this discussion in order to choose the best option
> > and make everyone agree on it.
> 
> I see. Thanks for the reply. And here is a sample implementation:
> 
> @@ -814,12 +815,15 @@ static struct tegra_smmu *tegra_smmu_find(struct device_node *np)
>  }
>  
>  static int tegra_smmu_configure(struct tegra_smmu *smmu, struct device *dev,
> -				struct of_phandle_args *args)
> +				struct of_phandle_args *args, struct fwnode_handle *fwnode)
>  {
>  	const struct iommu_ops *ops = smmu->iommu.ops;
>  	int err;
>  
> -	err = iommu_fwspec_init(dev, &dev->of_node->fwnode, ops);
> +	if (!fwnode)
> +		return -ENOENT;
> +
> +	err = iommu_fwspec_init(dev, fwnode, ops);
>  	if (err < 0) {
>  		dev_err(dev, "failed to initialize fwspec: %d\n", err);
>  		return err;
> @@ -835,6 +839,19 @@ static int tegra_smmu_configure(struct tegra_smmu *smmu, struct device *dev,
>  	return 0;
>  }
>  
> +static struct device_node *tegra_smmu_find_pci_np(struct pci_dev *pci_dev)
> +{
> +	struct pci_bus *bus = pci_dev->bus;
> +	struct device *dev = &bus->dev;
> +
> +	while (!of_property_read_bool(dev->of_node, "iommus") && bus->parent) {
> +		dev = &bus->parent->dev;
> +		bus = bus->parent;
> +	}
> +
> +	return dev->of_node;
> +}

This seems like it's the equivalent of pci_get_host_bridge_device(). Can
you use that instead? I think you might use the parent of the host
bridge that's returned from that function, though.

Thierry

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