Re: [ARM SMMU] Dynamic StreamID allocation

[Jean-Philippe Brucker <jean-philippe.brucker@arm.com>]

On 13/05/2019 08:09, Pankaj Bansal wrote:
> Hi Jean,
> 
>> -----Original Message-----
>> From: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
>> Sent: Friday, 10 May, 2019 07:07 PM
>> To: Pankaj Bansal <pankaj.bansal@nxp.com>; Will Deacon
>> <will.deacon@arm.com>; Robin Murphy <robin.murphy@arm.com>; Joerg
>> Roedel <joro@8bytes.org>
>> Cc: iommu@lists.linux-foundation.org; Varun Sethi <V.Sethi@nxp.com>; linux-
>> arm-kernel@lists.infradead.org; Nipun Gupta <nipun.gupta@nxp.com>
>> Subject: Re: [ARM SMMU] Dynamic StreamID allocation
>>
>> On 10/05/2019 13:33, Pankaj Bansal wrote:
>>> Hi Will/Robin/Joerg,
>>>
>>> I am s/w engineer from NXP India Pvt. Ltd.
>>> We are using SMMU-V3 in one of NXP SOC.
>>> I have a question about the SMMU Stream ID allocation in linux.
>>>
>>> Right now the Stream IDs allocated to a device are mapped via device tree to
>> the device.
>>> https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Felix
>>> ir.bootlin.com%2Flinux%2Flatest%2Fsource%2FDocumentation%2Fdevicetree%
>>> 2Fbindings%2Fiommu%2Farm%2Csmmu-
>> v3.txt%23L39&amp;data=02%7C01%7Cpankaj
>>> .bansal%40nxp.com%7C3cbe8bd4827e425afd0f08d6d54c925e%7C686ea1d3b
>> c2b4c6
>>>
>> fa92cd99c5c301635%7C0%7C0%7C636930922220665343&amp;sdata=vIG5u5n
>> XR5iRp
>>> uuuGjeFxKBtA5f5ohf91znXX0QWm1c%3D&amp;reserved=0
>>>
>>> As the device tree is passed from bootloader to linux, we detect all the stream
>> IDs needed by a device in bootloader and add their IDs in respective device
>> nodes.
>>> For each PCIE Endpoint (a unique BDF (Bus Device Function)) on PCIE bus, we
>> are assigning a unique Stream ID in bootloader.
>>>
>>> However, this poses an issue with PCIE hot plug.
>>> If we plug in a pcie device while linux is running, a unique BDF is assigned to
>> the device, for which there is no stream ID in device tree.
>>>
>>> How can this problem be solved in linux?
>>
>> Assuming the streamID associated to a BDF is predictable (streamID = BDF
>> + constant), using the iommu-map property should just work:
>>
>> https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Felixir.boo
>> tlin.com%2Flinux%2Flatest%2Fsource%2FDocumentation%2Fdevicetree%2Fbind
>> ings%2Fpci%2Fpci-
>> iommu.txt&amp;data=02%7C01%7Cpankaj.bansal%40nxp.com%7C3cbe8bd482
>> 7e425afd0f08d6d54c925e%7C686ea1d3bc2b4c6fa92cd99c5c301635%7C0%7C0
>> %7C636930922220665343&amp;sdata=GkkovEnvhd5dN%2BGdh%2FnKCyW5Cd
>> EnLDP3cWTrk%2B%2FO7EQ%3D&amp;reserved=0
>>
>> It describes the streamIDs of all possible BDFs, including hotplugged functions.
> 
> You mean that we should increase the "length" parameter (in (rid-base,iommu,iommu-base,length) touple) ?
> This would cater to any *new* Bus Device Function being detected on PCIE bus?
> Is that right ?

No, the iommu-map solution only works when you can predict at boot time
which streamID will correspond to any BDF, for example if the PCIe
controller or firmware automatically assigns streamID = BDF.

> Right now when we make iommu-map in bootloader, we are giving one RID per BDF:
> https://elixir.bootlin.com/u-boot/latest/source/drivers/pci/pcie_layerscape_fixup.c#L168

Right, that won't work with hotplug. You can't derive a unique streamID
from any BDF at boot, if the streamID range is limited to 16 values on
this hardware.

Maybe you could move this allocator to the Linux Layerscape driver, and
call iommu_fwspec_add_ids() from there, rather than using iommu-map? Not
sure how feasible that is, but it might still be the simplest.

Thanks,
Jean

> 
> But isn't the better approach to make it dynamic in linux?
> i.e. as soon as a new device is detected "requester id" is allocated to it from available pool.
> When device is removed, return the "requester id" to pool.
> is there any h/w limitation which prevents it?
> 
> Regards,
> Pankaj Bansal
>>
>> Thanks,
>> Jean
>>
>>>
>>> Is there a way to assign (and revoke) stream IDs at run time?
>>>
>>> Regards,
>>> Pankaj Bansal
>>>
>>>
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