linux-kernel - Re: [PATCH v5 2/3] iommu/tegra-smmu: Rework tegra_smmu_probe

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Date:   Mon, 5 Oct 2020 18:14:34 -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 v5 2/3] iommu/tegra-smmu: Rework tegra_smmu_probe_device()

On Mon, Oct 05, 2020 at 12:56:38PM +0200, Thierry Reding wrote:
> On Mon, Oct 05, 2020 at 11:41:08AM +0300, Dmitry Osipenko wrote:
> > 05.10.2020 00:57, Nicolin Chen пишет:
> > > On Sat, Oct 03, 2020 at 05:06:42PM +0300, Dmitry Osipenko wrote:
> > >> 03.10.2020 09:59, 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];
> > >>>  
> > >>> +	put_device(&iommu_pdev->dev);
> > >>> +
> > >>> +	if (!mc || !mc->smmu)
> > >>> +		return -EPROBE_DEFER;
> > >>
> > >> I'm not very excited by seeing code in the patches that can't be
> > >> explained by the patch authors and will appreciate if you could provide
> > >> a detailed explanation about why this NULL checking is needed because I
> > >> think it is unneeded, especially given that other IOMMU drivers don't
> > >> have such check.
> > > 
> > > This function could be called from of_iommu_configure(), which is
> > > a part of other driver's probe(). So I think it's safer to have a
> > > check. Yet, given mc driver is added to the "arch_initcall" stage,
> > > you are probably right that there's no really need at this moment
> > > because all clients should be called after mc/smmu are inited. So
> > > I'll resend a v6, if that makes you happy.
> > 
> > I wanted to get the explanation :) I'm very curious why it's actually
> > needed because I'm not 100% sure whether it's not needed at all.
> > 
> > I'd assume that the only possible problem could be if some device is
> > created in parallel with the MC probing and there is no locking that
> > could prevent this in the drivers core. It's not apparent to me whether
> > this situation could happen at all in practice.
> > 
> > The MC is created early and at that time everything is sequential, so
> > it's indeed should be safe to remove the check.
> 
> I think I now remember exactly why the "hack" in tegra_smmu_probe()
> exists. The reason is that the MC driver does this:
> 
> 	mc->smmu = tegra_smmu_probe(...);
> 
> That means that mc->smmu is going to be NULL until tegra_smmu_probe()
> has finished. But tegra_smmu_probe() calls bus_set_iommu() and that in
> turn calls ->probe_device(). So the purpose of the "hack" in the
> tegra_smmu_probe() function was to make sure mc->smmu was available at
> that point, because, well, it is already known, but we haven't gotten
> around to storing it yet.

Yea, that's exactly what I described in the commit message of a
later version of this patch. Yet, we can drop it now as a device
will probe() and call ->probe_device() again.

> ->of_xlate() can theoretically be called as early as right after
> bus_set_iommu() via of_iommu_configure() if that is called in parallel
> with tegra_smmu_probe(). I think that's very unlikely, but I'm not 100%
> sure that it can't happen.

As my previous reply to Dmitry above, I don't think it'd happen,
given that mc/smmu drivers are inited during arch_initcall stage
while all clients should be probed during device_initcall stage,
once this rework patch gets merged.

> In any case, I do agree with Dmitry that we should have a comment here
> explaining why this is necessary. Even if we're completely certain that
> this is necessary, it's not obvious and therefore should get that
> comment. And if we're not certain that it's necessary, it's probably
> also good to mention that in the comment so that eventually it can be
> determined or the check removed if it proves to be unnecessary.

Well, I have answered him, and also removed the mc/smmu check in
v6 already.