linux-kernel - Re: [PATCH v4 1/3] iommu/tegra-smmu: Use fwspec in tegra_smmu_(de)attach

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Date:   Fri, 2 Oct 2020 16:53:30 -0700
From:   Nicolin Chen <nicoleotsuka@...il.com>
To:     Dmitry Osipenko <digetx@...il.com>
Cc:     thierry.reding@...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 1/3] iommu/tegra-smmu: Use fwspec in
 tegra_smmu_(de)attach_dev

On Fri, Oct 02, 2020 at 11:12:18PM +0300, Dmitry Osipenko wrote:
> 02.10.2020 22:45, Nicolin Chen пишет:
> > On Fri, Oct 02, 2020 at 05:41:50PM +0300, Dmitry Osipenko wrote:
> >> 02.10.2020 09:08, Nicolin Chen пишет:
> >>>  static int tegra_smmu_attach_dev(struct iommu_domain *domain,
> >>>  				 struct device *dev)
> >>>  {
> >>> +	struct iommu_fwspec *fwspec = dev_iommu_fwspec_get(dev);
> >>>  	struct tegra_smmu *smmu = dev_iommu_priv_get(dev);
> >>>  	struct tegra_smmu_as *as = to_smmu_as(domain);
> >>> -	struct device_node *np = dev->of_node;
> >>> -	struct of_phandle_args args;
> >>>  	unsigned int index = 0;
> >>>  	int err = 0;
> >>>  
> >>> -	while (!of_parse_phandle_with_args(np, "iommus", "#iommu-cells", index,
> >>> -					   &args)) {
> >>> -		unsigned int swgroup = args.args[0];
> >>> -
> >>> -		if (args.np != smmu->dev->of_node) {
> >>> -			of_node_put(args.np);
> >>> -			continue;
> >>> -		}
> >>> -
> >>> -		of_node_put(args.np);
> >>> +	if (!fwspec)
> >>> +		return -ENOENT;
> >>
> >> Could the !fwspec ever be true here as well?
> > 
> > There are multiple callers of this function. It's really not that
> > straightforward to track every one of them. So I'd rather have it
> > here as other iommu drivers do. We are human beings, so we could
> > have missed something somewhere, especially callers are not from
> > tegra-* drivers.
> > 
> 
> I'm looking at the IOMMU core and it requires device to be in IOMMU
> group before attach_dev() could be called.
> 
> The group can't be assigned to device without the fwspec, see
> tegra_smmu_device_group().
>
> Seems majority of IOMMU drivers are checking dev_iommu_priv_get() for
> NULL in attach_dev(), some not checking anything, some check both and
> only arm-smmu checks the fwspec.

As I said a couple of days ago, I don't like to assume that the
callers won't change. And this time, it's from open code. So I
don't want to assume that there won't be a change.

If you are confident that there is no need to add such a check,
please send patches to remove those checks in those drivers to
see if others would agree. I would be willing to remove it after
that. Otherwise, I'd like to keep this.

Thanks for the review.