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Date:   Mon, 19 Oct 2020 11:55:41 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Nicolin Chen <nicoleotsuka@...il.com>
Cc:     thierry.reding@...il.com, joro@...tes.org, digetx@...il.com,
        linux-tegra@...r.kernel.org, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org, jonathanh@...dia.com
Subject: Re: [PATCH v7 3/3] iommu/tegra-smmu: Add PCI support

On 2020-10-17 02:56, Nicolin Chen wrote:
> On Fri, Oct 16, 2020 at 03:10:26PM +0100, Robin Murphy wrote:
>> On 2020-10-16 04:53, Nicolin Chen wrote:
>>> On Thu, Oct 15, 2020 at 10:55:52AM +0100, Robin Murphy wrote:
>>>> On 2020-10-15 05:13, Nicolin Chen wrote:
>>>>> On Wed, Oct 14, 2020 at 06:42:36PM +0100, Robin Murphy wrote:
>>>>>> On 2020-10-09 17:19, Nicolin Chen wrote:
>>>>>>> This patch simply adds support for PCI devices.
>>>>>>>
>>>>>>> Reviewed-by: Dmitry Osipenko <digetx@...il.com>
>>>>>>> Tested-by: Dmitry Osipenko <digetx@...il.com>
>>>>>>> Signed-off-by: Nicolin Chen <nicoleotsuka@...il.com>
>>>>>>> ---
>>>>>>>
>>>>>>> Changelog
>>>>>>> v6->v7
>>>>>>>      * Renamed goto labels, suggested by Thierry.
>>>>>>> v5->v6
>>>>>>>      * Added Dmitry's Reviewed-by and Tested-by.
>>>>>>> v4->v5
>>>>>>>      * Added Dmitry's Reviewed-by
>>>>>>> v3->v4
>>>>>>>      * Dropped !iommu_present() check
>>>>>>>      * Added CONFIG_PCI check in the exit path
>>>>>>> v2->v3
>>>>>>>      * Replaced ternary conditional operator with if-else in .device_group()
>>>>>>>      * Dropped change in tegra_smmu_remove()
>>>>>>> v1->v2
>>>>>>>      * Added error-out labels in tegra_smmu_probe()
>>>>>>>      * Dropped pci_request_acs() since IOMMU core would call it.
>>>>>>>
>>>>>>>      drivers/iommu/tegra-smmu.c | 35 +++++++++++++++++++++++++----------
>>>>>>>      1 file changed, 25 insertions(+), 10 deletions(-)
>>>>>>>
>>>>>>> diff --git a/drivers/iommu/tegra-smmu.c b/drivers/iommu/tegra-smmu.c
>>>>>>> index be29f5977145..2941d6459076 100644
>>>>>>> --- a/drivers/iommu/tegra-smmu.c
>>>>>>> +++ b/drivers/iommu/tegra-smmu.c
>>>>>>> @@ -10,6 +10,7 @@
>>>>>>>      #include <linux/kernel.h>
>>>>>>>      #include <linux/of.h>
>>>>>>>      #include <linux/of_device.h>
>>>>>>> +#include <linux/pci.h>
>>>>>>>      #include <linux/platform_device.h>
>>>>>>>      #include <linux/slab.h>
>>>>>>>      #include <linux/spinlock.h>
>>>>>>> @@ -865,7 +866,11 @@ static struct iommu_group *tegra_smmu_device_group(struct device *dev)
>>>>>>>      	group->smmu = smmu;
>>>>>>>      	group->soc = soc;
>>>>>>> -	group->group = iommu_group_alloc();
>>>>>>> +	if (dev_is_pci(dev))
>>>>>>> +		group->group = pci_device_group(dev);
>>>>>>
>>>>>> Just to check, is it OK to have two or more swgroups "owning" the same
>>>>>> iommu_group if an existing one gets returned here? It looks like that might
>>>>>> not play nice with the use of iommu_group_set_iommudata().
>>>>>
>>>>> Do you mean by "gets returned here" the "IS_ERR" check below?
>>>>
>>>> I mean that unlike iommu_group_alloc()/generic_device_group(),
>>>> pci_device_group() may give you back a group that already contains another
>>>> device and has already been set up from that device's perspective. This can
>>>> happen for topological reasons like requester ID aliasing through a PCI-PCIe
>>>> bridge or lack of isolation between functions.
>>>
>>> Okay..but we don't really have two swgroups owning the same groups
>>> in case of PCI devices. For Tegra210, all PCI devices inherit the
>>> same swgroup from the PCI controller. And I'd think previous chips
>>> do the same. The only use case currently of 2+ swgroups owning the
>>> same iommu_group is for display controller.
>>>
>>> Or do you suggest we need an additional check for pci_device_group?
>>
>> Ah, OK - I still don't have the best comprehension of what exactly swgroups
> 
> The "swgroup" stands for "software group", which associates with
> an ASID (Address Space Identifier) for SMMU to determine whether
> this client is going through SMMU translation or not.

So in Arm SMMU analogy terms it's more like a context bank index than a 
stream ID - got it.

>> are, and the path through .of_xlate looked like you might be using the PCI
>> requester ID as the swgroup identifier, but I guess that ultimately depends
>> on what your "iommu-map" is supposed to look like. If pci_device_group()
> 
> This is copied from pcie node in our downstream dtsi file:
> 
> 		iommus = <&mc TEGRA_SWGROUP_AFI>;
> 		iommu-map = <0x0 &mc TEGRA_SWGROUP_AFI 0x1000>;
> 		iommu-map-mask = <0x0>;

Aha, indeed that iommu-map-mask is the trick :)

>> will effectively only ever get called once regardless of how many endpoints
>> exist, then indeed this won't be a concern (although if that's *guaranteed*
>> to be the case then you may as well just stick with calling
>> iommu_group_alloc() directly). Thanks for clarifying.
> 
> All PCI devices are supposed to get this swgroup of the pcie node
> above. So the function will return the existing group of the pcie
> controller, before giving a chance to call iommu_group_alloc().

Yes, the "iommus" property will mean that the group always gets created 
first for the platform device owning the host bridge, and thus won't be 
visible to pci_device_group() anyway. Subsequent tegra_smmu_group_get() 
calls for the PCI devices (including the PCI side of the host bridge 
itself) are then going to match TEGRA_SWGROUP_AFI in the smmu->groups 
list lookup and return early, so the dev_is_pci() condition will never 
be true, and the call to pci_device_group() is in fact entirely dead code.

(I was assuming a case where you didn't have the "iommus" property, in 
which case you would reach that path exactly once for the first PCI 
device probed, wherein pci_device_group() is still only going to fall 
through to calling iommu_group_alloc() anyway).

Cheers,
Robin.

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