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

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Message-ID: <b966844e-4289-3ff0-9512-852f8419a664@gmail.com>
Date:   Thu, 1 Oct 2020 23:33:38 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Nicolin Chen <nicoleotsuka@...il.com>,
        Thierry Reding <thierry.reding@...il.com>
Cc:     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

01.10.2020 14:04, Nicolin Chen пишет:
> 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...

Not very convincing to me. I don't see a "plenty of other drivers",
there is only one arm-smmu driver.

The dev_get_dev_node() is under CONFIG_ARM_SMMU_LEGACY_DT_BINDINGS (!).
Guys, doesn't it look strange to you? :)

The arm-smmu driver does a similar thing for the modern bindings to what
Nicolin's v3 is doing.

>>> 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().

You don't need the global pointer if you have SMMU OF node.

You could also get driver pointer from mc->dev->driver.

But I don't think you need to do this at all. The probe_device() could
be invoked only for the tegra_smmu_ops and then seems you could use
dev_iommu_priv_set() in tegra_smmu_of_xlate(), like sun50i-iommu driver
does.