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

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Date:   Fri, 2 Oct 2020 04:55:34 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Nicolin Chen <nicoleotsuka@...il.com>
Cc:     Thierry Reding <thierry.reding@...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

02.10.2020 04:07, Nicolin Chen пишет:
> On Thu, Oct 01, 2020 at 11:33:38PM +0300, Dmitry Osipenko wrote:
>>>>> 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.
> 
> Getting iommu device pointer using driver_find_device_by_fwnode()
> is a common practice in ->probe_device() of other iommu drivers.

Please give me a full list of the IOMMU drivers which use this method.

> But this requires a device_driver pointer that tegra-smmu doesn't
> have. So passing tegra_mc_driver through tegra_smmu_probe() will
> address it.
> 

If you're borrowing code and ideas from other drivers, then at least
please borrow them from a modern good-looking drivers. And I already
pointed out that following cargo cult is not always a good idea.

ARM-SMMU isn't a modern driver and it has legacy code. You shouldn't
copy it blindly. The sun50i-iommu driver was added half year ago, you
may use it as a reference.

Always consult the IOMMU core code. If you're too unsure about
something, then maybe better to start a new thread and ask Joerg about
the best modern practices that IOMMU drivers should use.