Subject: [PATCH v5] devicetree: Add generic IOMMU device tree bindings

From: [email-masked] (Hiroshi Doyu)
Date: Tue, 19 Aug 2014 13:03:52 +0300

Varun Sethi <[email-masked]> writes:

> Hi Hiroshi,
>
>> -----Original Message-----
>> From: Hiroshi Doyu [mailto:hdoyu at nvidia.com]
>> Sent: Thursday, August 14, 2014 9:35 PM
>> To: Sethi Varun-B16395
>> Cc: Hiroshi Doyu; Thierry Reding; Stephen Warren; Arnd Bergmann; Will
>> Deacon; Mark Rutland; devicetree at vger.kernel.org; Olof Johansson;
>> iommu at lists.linux-foundation.org; Rob Herring; linux-tegra at vger.kernel.org;
>> linux-arm-kernel at lists.infradead.org
>> Subject: Re: [PATCH v5] devicetree: Add generic IOMMU device tree bindings
>> 
>> Hi Varun,
>> 
>> Varun Sethi <[email-masked]> writes:
>> 
>> >> -----Original Message-----
>> >> From: iommu-bounces at lists.linux-foundation.org [mailto:iommu-
>> >> bounces at lists.linux-foundation.org] On Behalf Of Hiroshi Doyu
>> >> Sent: Thursday, August 14, 2014 12:18 PM
>> >> To: Thierry Reding; Stephen Warren; Arnd Bergmann; Will Deacon
>> >> Cc: Mark Rutland; devicetree at vger.kernel.org; Olof Johansson;
>> >> iommu at lists.linux-foundation.org; Rob Herring;
>> >> linux-tegra at vger.kernel.org; linux-arm-kernel at lists.infradead.org
>> >> Subject: Re: [PATCH v5] devicetree: Add generic IOMMU device tree
>> >> bindings
>> >>
>> >>
>> >> Thierry Reding <[email-masked]> writes:
>> >>
>> >> > +Multiple-master IOMMU:
>> >> > +----------------------
>> >> > +
>> >> > +       iommu {
>> >> > +               /* the specifier represents the ID of the master */
>> >> > +               #iommu-cells = <1>;
>> >> > +       };
>> >> > +
>> >> > +       master at 1 {
>> >> > +               /* device has master ID 42 in the IOMMU */
>> >> > +               iommus = <&{/iommu} 42>;
>> >> > +       };
>> >> > +
>> >> > +       master at 2 {
>> >> > +               /* device has master IDs 23 and 24 in the IOMMU */
>> >> > +               iommus = <&{/iommu} 23>, <&{/iommu} 24>;
>> >> > +       };
>> >>
>> >> I think that this "master ID" will be parsed in IOMMU driver. For
>> >> example, ARM,SMMU expects "streamID" as "master ID", right?
>> >>
>> >> If a SoC has a feature to configure to assign streamID to devices at
>> >> runtime, "streamID" is not equal to "master ID".
>> >>
>> >>   iommus = <&{/smmu} "soc specific master ID">;
>> >>
>> >> "soc master ID" needs to be translated into "streamID" by SoC SW. It
>> >> seems that ARM,SMMU kernel driver doesn't expect this kind of ID
>> >> translation. If ARM,SMMU kernel driver is used as is, "soc master ID"
>> >> would be incompatible? ARM,SMMU needs such translation before
>> >> parsing. Is this my understanding right?
>> >>
>> >> If so I think that this master ID configuration/translation may be
>> >> quite reasonable requirment for SoC using ARM,SMMU.
>> >>
>> >> Can we consider this ID translation within ARM,SMMU compatibility?
>> >>
>> >> IOW, is it possible to implement some SoC specific hook for ID
>> >> translation/configuration in ARM,SMMU kernel driver?
>> >
>> >
>> > Can the id translation be done using a SMR mask?
>> 
>> No, "SoC master ID" is completely independenf of SMR.
>> 
>> > Also, for dynamic stream ID allocation we would need to represent the
>> > specific master register (to store the stream ID) in the device tree.
>> 
>> I assmue that the above means that iMX has such configuration register to map
>> steramID and a device dynamically.
>
> We have per master registers for setting the stream ID on the
> Layerscape platforms. My point was that we would need the iommu master
> node to include a reference to the master id register.
>
> master at 1 {
>                /* device has master ID 42 in the IOMMU */
>              iommus = <&{/iommu} 42>;
>              master-id-reg = <phandle offset>
> };

In the above, for "iommus=" bindings, you wouldn't need to break
ARM,SMMU compatibility at all if you set "streamID" exactly as below.

  master at 1 {
                 /* device has master ID 42 in the IOMMU */
               iommus = <&{/iommu} 'any given streamID'>;
               master-id-reg = <phandle offset>
  };

And your SoC needs to register bus_notifier and ADD_DEVICE should
configure to map 'any given streamID' to a device via the above
register. This wouldn't need any modification from ARM,SMMU driver and
keep the iommus bindings as it is.

IOW, SoC only needs to register ADD_DEVICE in bus_notifier to map
StreamID to a device. This needs to be executed earlier than IOMMU bus's
ADD_DEVICE, though.

Is my understanding right?