Re: [Xen-devel] [PATCH v2 10/12] xen/iommu: smmu: Check for duplicate stream IDs when registering master devices

[Julien Grall <julien.grall@xxxxxxxxxx>]

On 27/01/15 17:44, Ian Campbell wrote:
> On Tue, 2015-01-27 at 17:33 +0000, Julien Grall wrote:
>> On 27/01/15 17:02, Stefano Stabellini wrote:
>>> On Tue, 27 Jan 2015, Julien Grall wrote:
>>>> On 27/01/15 16:30, Stefano Stabellini wrote:
>>>>> On Fri, 16 Jan 2015, Julien Grall wrote:
>>>>>> From: Andreas Herrmann <andreas.herrmann@xxxxxxxxxxx>
>>>>>>
>>>>>> If DT information lists one stream ID twice for the master devices of
>>>>>> an SMMU this can cause a multi match when stream ID matching is used.
>>>>>> For stream ID indexing this might trigger an overwrite of an S2CR that
>>>>>> is already in use.
>>>>>>
>>>>>> So better check for duplicates when DT information is parsed.
>>>>>>
>>>>>> Taken from the linux ML:
>>>>>> http://lists.infradead.org/pipermail/linux-arm-kernel/2014-January/226099.html
>>>>>>
>>>>>> Cc: Andreas Herrmann <herrmann.der.user@xxxxxxxxxxxxxx>
>>>>>> Signed-off-by: Andreas Herrmann <andreas.herrmann@xxxxxxxxxxx>
>>>>>> Signed-off-by: Julien Grall <julien.grall@xxxxxxxxxx>
>>>>>
>>>>> Why didn't you just take a more recent version of the Linux smmu driver?
>>>>
>>>> The SMMU driver very is recent (see commit in the previous patch)...
>>>> Just this patch has never reached upstream.
>>>
>>> That is not good. It might be worth to wait for it to go upstream.
>>
>> The patch was sent one year ago. Just before Calxeda was shutdown.
>>
>> This is a requirement for the following patch. Do you think the other
>> patch should be upstream to Linux before?
> 
> In general we should be reluctant to diverge over these things, since it
> just ends up making things harder for us in the future (e.g. the next
> resync or the one after).
> 
> Is the issue being fixed here specific to Calxeda? e.g. is duplicate
> stream ids a h/w bug which has yet to be observed elsewhere or is it a
> valid configuration per the h/w specs which it just happens only Calxeda
> implemented?

It's for catching possible error in the device tree. If the ID is
duplicated the multi match algo in the following patch may not work
correctly.

> 
> Unless it's a h/w bug I don't see why it couldn't go upstream first.
> Even if it is it might still stand a chance.

Because nobody take care to upstream it (see why below).

>> If so, Calxeda server won't be able to use properly SMMU.
>>
>> Even though the server will never be used, I do all my SMMU development
>> on it.
> 
> The fact that Calxeda are no longer around doesn't in itself preclude
> getting these patches upstreamed. After all, no one is deleting CX
> support and some people do have them (not just us). It's hardly the most
> obscure platform supported by Linux...

By default Linux is by-passing the stream ID and the device is not using
the SMMU.

You will have to use either passthrough or manually said : "I want to
enforce this device". And then you will see the error. So by default
both patches are not necessary.

On Xen, we can't by-pass the SMMU by default. At least it's not the way
it should work.

Regards,

-- 
Julien Grall

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