Re: [RFC PATCH 0/3] Introduce iommu-map-masked for platform devices
From: Robin Murphy
Date: Mon Oct 13 2025 - 07:21:04 EST
On 2025-10-09 7:25 pm, Dmitry Baryshkov wrote:
On Thu, Oct 09, 2025 at 06:03:29PM +0100, Robin Murphy wrote:
On 2025-10-09 2:19 pm, Dmitry Baryshkov wrote:
On Thu, Oct 09, 2025 at 11:46:55AM +0100, Robin Murphy wrote:
On 2025-10-08 8:10 pm, Charan Teja Kalla wrote:
On 9/29/2025 3:50 PM, Robin Murphy wrote:
USECASE [1]:
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Video IP, 32bit, have 2 hardware sub blocks(or can be called as
functions) called as pixel and nonpixel blocks, that does decode and
encode of the video stream. These sub blocks are __configured__ to
generate different stream IDs.
So please clarify why you can't:
a) Describe the sub-blocks as individual child nodes each with their own
distinct "iommus" property
Thanks Robin for your time. Sorry for late reply as I really didn't have
concrete answer for this question.
First let me clarify the word "sub blocks" -- This is just the logical
separation with no separate address space to really able to define them
as sub devices. Think of it like a single video IP with 2 dma
engines(used for pixel and non-pixel purpose).
I should agree that the child-nodes in the device tree is the easy one
and infact, it is how being used in downstream.
For upstream -- Since there is no real address space to interact with
these sub-blocks(or logical blocks), does it really qualify to define as
child nodes in the device tree? I see there is some push back[1].
Who says you need an address space? Child nodes without "reg" properties,
referenced by name, compatible or phandle, exist all over the place for all
manner of reasons. If there are distinct logical functions with their own
distinct hardware properties, then I would say having child nodes to
describe and associate those properties with their respective functions is
entirely natural and appropriate. The first example that comes to mind of
where this is a well-established practice is PMICs - to pick one at random:
Documentation/devicetree/bindings/regulator/qcom,rpmh-regulator.yaml
Logical function, that's correct. And also note, for PMICs that practice
has bitten us back. For PM8008 we switched back to a non-subdevice
representation.
For bonus irony, you can't take the other approaches without inherently
*introducing* a notional address space in the form of your logical function
IDs anyway.
> or:An IOMMU specifier is whatever the target IOMMU node's #iommu-cells says it
b) Use standard "iommu-map" which already supports mapping a masked
input ID to an arbitrary IOMMU specifier
I think clients is also required to program non-zero smr mask, where as
iommu-map just maps the id to an IOMMU specifier(sid). Please LMK if I
am unable to catch your thought here.
is. The fact that Linux's parsing code only works properly for #iommu-cells
= 1 is not really a DT binding problem (other than it stemming from a loose
assumption stated in the PCI binding's use of the property).
I really don't like the idea of extending the #iommu-cells. The ARM SMMU
has only one cell, which is correct even for our platforms. The fact
that we need to identify different IOMMU SIDs (and handle them in a
differnt ways) is internal to the video device (and several other
devices). There is nothing to be handled on the ARM SMMU side.
Huh? So if you prefer not to change anything, are you suggesting this series
doesn't need to exist at all? Now I'm thoroughly confused...
Hmm. We need changes, but I don't feel like adding the FUNCTION_ID to
#iommu-cells is the best idea.
What? No, any function ID would be an *input* to a map, not part of the output specifier; indeed it should never go anywhere near the IOMMU, I don't think anyone suggested that.
If you want to use SMR masks, then you absolutely need #iommu-cells = 2,
because that is the SMMU binding for using SMR masks. It would definitely
I'm sorry. Yes, we have #iommu-cells = <2>.
not be OK to have some magic property trying to smuggle
IOMMU-driver-specific data contrary to what the IOMMU node itself says. As
for iommu-map, I don't see what would be objectionable about improving the
parsing to respect a real #iommu-cells value rather than hard-coding an
assumption. Yes, we'd probably need to forbid entries with length > 1
targeting IOMMUs with #iommu-cells > 1, since the notion of a linear
This will break e.g. PCIe on Qualcomm platforms:
iommu-map = <0x0 &apps_smmu 0x1400 0x1>,
<0x100 &apps_smmu 0x1401 0x1>;
But this seems unlogical anyway wrt. apps_smmu having #iommu-cells =
<2>. It depends on ARM SMMU ignoring the second cell when it's not
present.
Urgh, yes, that's just broken already :(
At least they all seem to be a sufficiently consistent pattern that a targeted workaround to detect old DTBs looks feasible (I'm thinking, if iommu-map size % 4 == 0 and cells n*4 + 3 are all 1 and cells n*4 + 1 are all the same phandle to an IOMMU with #iommu-cells == 2, then parse as if #iommu-cells == 1)
No, the *display* driver does not; the host1x bus driver does, which is the point I was making - that has a proper bus abstraction tied into the IOMMU layer, such that the devices are correctly configured long before the actual DRM driver(s) get anywhere near them.relationship between the input ID and the output specifier falls apart when
the specifier is complex, but that seems simple enough to implement and
document (even if it's too fiddly to describe in the schema itself), and
still certainly no worse than having another property that *is* just
iommu-map with implicit length = 1.
And if you want individual StreamIDs for logical functions to be attachable
to distinct contexts then those functions absolutely must be visible to the
IOMMU layer and the SMMU driver as independent devices with their own unique
properties, which means either they come that way from the DT as of_platform
devices in the first place, or you implement a full bus_type abstraction
Not necessarily. Tegra display driver creates a device for each context
on its own.
In fact, using OF to create context devices is _less_
robust, because now the driver needs to sync, checking that there is a
subdevice, that it has probed, etc. Using manually created devices seems
better from my POV.
Huh? A simple call to of_platform_populate() is somehow less robust than open-coding much of the same logic that of_platform_populate() does plus a bunch of hackery to try to fake up an of_node to make the new device appear to own the appropriate properties?
Having entire sub-*drivers* for child devices or not is an orthogonal issue regardless of whichever way they are created.
I mean IOMMU configuration is designed to happen at device_add() time, and client drivers must not assume otherwise (the mechanisms for handling IOMMU drivers registering "late" from modules are internal details that can and will change). If you're under the impression that a straightforward platform driver for the video codec itself would be able to invoke IOMMU configuration for the video codec platform device (without unacceptable levels of hackery) then you are mistaken, sorry.which will have to be hooked up to the IOMMU layer. You cannot make IOMMU
configuration "internal" to the actual client driver which is only allowed
to bind *after* said IOMMU configuration has already been made.
I'm not sure I follow this, I'm sorry.
Again, to be able to assign StreamIDs to different contexts, those StreamIDs must uniquely belong to different struct devices. Thus in terms of how you get to those struct devices from a DT representation, either they come from distinct DT nodes with standard "iommus" properties that the generic of_platform code can create and configure accordingly, or you're doing a non-trivial amount of work to implement your own bus layer like host1x_context_bus to manage your own type of sub-device. There is no valid middle ground of trying to stuff driver-specific knowledge of arbitrarily made-up function IDs into the generic platform bus code.
Thanks,
Robin.