linux-kernel - Re: [PATCH v4 2/3] iommu/tegra-smmu: Rework tegra_smmu_probe

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Message-ID: <39cb0056-1447-2232-d33c-adb17114740a@gmail.com>
Date:   Mon, 5 Oct 2020 16:28:53 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Thierry Reding <thierry.reding@...il.com>
Cc:     Nicolin Chen <nicoleotsuka@...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 2/3] iommu/tegra-smmu: Rework tegra_smmu_probe_device()

05.10.2020 14:15, Thierry Reding пишет:
> On Mon, Oct 05, 2020 at 01:36:55PM +0300, Dmitry Osipenko wrote:
>> 05.10.2020 12:53, Thierry Reding пишет:
>>> On Fri, Oct 02, 2020 at 05:50:08PM +0300, Dmitry Osipenko wrote:
>>>> 02.10.2020 17:22, Dmitry Osipenko 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];
>>>>>>  
>>>>>> +	of_node_put(args->np);
>>>>>> +
>>>>>> +	if (!mc || !mc->smmu)
>>>>>> +		return -EPROBE_DEFER;
>>>>> platform_get_drvdata(NULL) will crash.
>>>>>
>>>>
>>>> Actually, platform_get_drvdata(NULL) can't happen. I overlooked this.
>>>
>>> How so? It's technically possible for the iommus property to reference a
>>> device tree node for which no platform device will ever be created, in
>>> which case of_find_device_by_node() will return NULL. That's very
>>> unlikely and perhaps worth just crashing on to make sure it gets fixed
>>> immediately.
>>
>> The tegra_smmu_ops are registered from the SMMU driver itself and MC
>> driver sets platform data before SMMU is initialized, hence device is
>> guaranteed to exist and mc can't be NULL.
> 
> Yes, but that assumes that args->np points to the memory controller's
> device tree node. It's obviously a mistake to do this, but I don't think
> anyone will prevent you from doing this:
> 
> 	iommus = <&{/chosen} 0>;
> 
> In that case, since no platform device is created for the /chosen node,
> iommu_pdev will end up being NULL and platform_get_drvdata() will crash.

But then Tegra SMMU isn't associated with the device's IOMMU path, and
thus, tegra_smmu_of_xlate() won't be invoked for this device.

> That said, I'm fine with not adding a check for that. If anyone really
> does end up messing this up they deserve the crash.
> 
> I'm still a bit undecided about the mc->smmu check because I haven't
> convinced myself yet that it can't happen.

For now I can't see any realistic situation where mc->smmu could be NULL.