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

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Date:   Thu, 1 Oct 2020 04:04:25 -0700
From:   Nicolin Chen <nicoleotsuka@...il.com>
To:     Thierry Reding <thierry.reding@...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 Thu, Oct 01, 2020 at 12:23:16PM +0200, Thierry Reding 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 personally am not a fan of adding a path for PCI device either,
> > since PCI/IOMMU cores could have taken care of it while the same
> > path can't be used for other buses.
> 
> There's already plenty of other drivers that do something similar to
> this. Take a look at the arm-smmu driver, for example, which seems to be
> doing exactly the same thing to finding the right device tree node to
> look at (see dev_get_dev_node() in drivers/iommu/arm-smmu/arm-smmu.c).

Hmm..okay..that is quite convincing then...

> > If we can't come to an agreement on globalizing mc pointer, would
> > it be possible to pass tegra_mc_driver through tegra_smmu_probe()
> > so we can continue to use driver_find_device_by_fwnode() as v1?
> > 
> > v1: https://lkml.org/lkml/2020/9/26/68
> 
> tegra_smmu_probe() already takes a struct tegra_mc *. Did you mean
> tegra_smmu_probe_device()? I don't think we can do that because it isn't

I was saying to have a global parent_driver pointer: similar to
my v1, yet rather than "extern" the tegra_mc_driver, we pass it
through egra_smmu_probe() and store it in a static global value
so as to call tegra_smmu_get_by_fwnode() in ->probe_device().

Though I agree that creating a global device pointer (mc) might
be controversial, yet having a global parent_driver pointer may
not be against the rule, considering that it is common in iommu
drivers to call driver_find_device_by_fwnode in probe_device().

> known at that point whether MC really is the SMMU. That's in fact the
> whole reason why we have to go through this whole dance of iterating
> over the iommus entries to find the SMMU.

Hmm..I don't quite get the meaning of:
"it isn't known at that point whether MC really is the SMMU".

Are you saying the stage of bus_set_iommu()? So because at that
point either SMMU probe() or MC probe() hasn't finished yet?

Thanks!