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Subject: Re: [PATCH v3 2/3] iommu/tegra-smmu: Rework .probe_device and
 .attach_dev
To:     Nicolin Chen <nicoleotsuka@gmail.com>
Cc:     Thierry Reding <thierry.reding@gmail.com>, joro@8bytes.org,
        krzk@kernel.org, vdumpa@nvidia.com, jonathanh@nvidia.com,
        linux-tegra@vger.kernel.org, iommu@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org
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From:   Dmitry Osipenko <digetx@gmail.com>
Message-ID: <aad6f530-6b62-b75d-44ad-0376650efc86@gmail.com>
Date:   Thu, 1 Oct 2020 07:04:59 +0300
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01.10.2020 05:48, Nicolin Chen пишет:
> On Thu, Oct 01, 2020 at 05:06:19AM +0300, Dmitry Osipenko wrote:
>> 01.10.2020 04:26, Nicolin Chen пишет:
>>> On Thu, Oct 01, 2020 at 12:56:46AM +0300, Dmitry Osipenko wrote:
>>>> 01.10.2020 00:32, Nicolin Chen пишет:
>>>>> On Thu, Oct 01, 2020 at 12:24:25AM +0300, Dmitry Osipenko 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'm not sure exactly what find_host_bridge() is called, but I'm pretty
>>>>>>>> sure that exists.
>>>>>>>>
>>>>>>>> Once we have that we can still iterate over the iommus property and do
>>>>>>>> not need to rely on this global variable.
>>>>>>>
>>>>>>> I agree that it'd work. But I was hoping to simplify the code
>>>>>>> here if it's possible. Looks like we have an argument on this
>>>>>>> so I will choose to go with your suggestion above for now.
>>>>>>
>>>>>> This patch removed more lines than were added. If this will be opposite
>>>>>> for the Thierry's suggestion, then it's probably not a great suggestion.
>>>>>
>>>>> Sorry, I don't quite understand this comments. Would you please
>>>>> elaborate what's this "it" being "not a great suggestion"?
>>>>>
>>>>
>>>> I meant that you should try to implement Thierry's solution, but if the
>>>> end result will be worse than the current patch, then you shouldn't make
>>>> a v4, but get back to this discussion in order to choose the best option
>>>> and make everyone agree on it.
>>>
>>> I see. Thanks for the reply. And here is a sample implementation:
>>
>> That's what I supposed to happen :) The new variant adds code and
>> complexity, while old did the opposite. Hence the old variant is clearly
>> more attractive, IMO.
> 
> 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.
> 
> 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
> 

I think we already agreed that it will be good to have a common helper.
So far Thierry only objected that the implementation of the helper could
be improved.