New subject: [PATCH v2 1/5] dt-bindings: reserved-memory: Document memory region specifier

1 Sep 2021


      On Fri, Jul 02, 2021 at 05:16:25PM +0300, Dmitry Osipenko wrote:
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01.07.2021 21:14, Thierry Reding пишет:
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On Tue, Jun 08, 2021 at 06:51:40PM +0200, Thierry Reding wrote:
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On Fri, May 28, 2021 at 06:54:55PM +0200, Thierry Reding wrote:
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On Thu, May 20, 2021 at 05:03:06PM -0500, Rob Herring wrote:
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On Fri, Apr 23, 2021 at 06:32:30PM +0200, Thierry Reding wrote:
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From: Thierry Reding treding@nvidia.com
Reserved memory region phandle references can be accompanied by a
specifier that provides additional information about how that specific
reference should be treated.
One use-case is to mark a memory region as needing an identity mapping
in the system's IOMMU for the device that references the region. This is
needed for example when the bootloader has set up hardware (such as a
display controller) to actively access a memory region (e.g. a boot
splash screen framebuffer) during boot. The operating system can use the
identity mapping flag from the specifier to make sure an IOMMU identity
mapping is set up for the framebuffer before IOMMU translations are
enabled for the display controller.
Signed-off-by: Thierry Reding treding@nvidia.com
.../reserved-memory/reserved-memory.txt       | 21 +++++++++++++++++++
 include/dt-bindings/reserved-memory.h         |  8 +++++++
 2 files changed, 29 insertions(+)
 create mode 100644 include/dt-bindings/reserved-memory.h
Sorry for being slow on this. I have 2 concerns.
First, this creates an ABI issue. A DT with cells in 'memory-region' 
will not be understood by an existing OS. I'm less concerned about this 
if we address that with a stable fix. (Though I'm pretty sure we've 
naively added #?-cells in the past ignoring this issue.)
A while ago I had proposed adding memory-region*s* as an alternative
name for memory-region to make the naming more consistent with other
types of properties (think clocks, resets, gpios, ...). If we added
that, we could easily differentiate between the "legacy" cases where
no #memory-region-cells was allowed and the new cases where it was.
...
Second, it could be the bootloader setting up the reserved region. If a 
node already has 'memory-region', then adding more regions is more 
complicated compared to adding new properties. And defining what each 
memory-region entry is or how many in schemas is impossible.
It's true that updating the property gets a bit complicated, but it's
not exactly rocket science. We really just need to splice the array. I
have a working implemention for this in U-Boot.
For what it's worth, we could run into the same issue with any new
property that we add. Even if we renamed this to iommu-memory-region,
it's still possible that a bootloader may have to update this property
if it already exists (it could be hard-coded in DT, or it could have
been added by some earlier bootloader or firmware).
...
Both could be addressed with a new property. Perhaps something like 
'iommu-memory-region = <&phandle>;'. I think the 'iommu' prefix is 
appropriate given this is entirely because of the IOMMU being in the 
mix. I might feel differently if we had other uses for cells, but I 
don't really see it in this case.
I'm afraid that down the road we'll end up with other cases and then we
might proliferate a number of *-memory-region properties with varying
prefixes.
I am aware of one other case where we might need something like this: on
some Tegra SoCs we have audio processors that will access memory buffers
using a DMA engine. These processors are booted from early firmware
using firmware from system memory. In order to avoid trashing the
firmware, we need to reserve memory. We can do this using reserved
memory nodes. However, the audio DMA engine also uses the SMMU, so we
need to make sure that the firmware memory is marked as reserved within
the SMMU. This is similar to the identity mapping case, but not exactly
the same. Instead of creating a 1:1 mapping, we just want that IOVA
region to be reserved (i.e. IOMMU_RESV_RESERVED instead of
IOMMU_RESV_DIRECT{,_RELAXABLE}).
That would also fall into the IOMMU domain, but we can't reuse the
iommu-memory-region property for that because then we don't have enough
information to decide which type of reservation we need.
We could obviously make iommu-memory-region take a specifier, but we
could just as well use memory-regions in that case since we have
something more generic anyway.
With the #memory-region-cells proposal, we can easily extend the cell in
the specifier with an additional MEMORY_REGION_IOMMU_RESERVE flag to
take that other use case into account. If we than also change to the new
memory-regions property name, we avoid the ABI issue (and we gain a bit
of consistency while at it).
Ping? Rob, do you want me to add this second use-case to the patch
series to make it more obvious that this isn't just a one-off thing? Or
how do we proceed?
Rob, given that additional use-case, do you want me to run with this
proposal and send out an updated series?
What about variant with a "descriptor" properties that will describe
each region:
fb_desc: display-framebuffer-memory-descriptor {
   needs-identity-mapping;
}
display@52400000 {
   memory-region = <&fb ...>;
   memory-region-descriptor = <&fb_desc ...>;
};
It could be a more flexible/extendible variant.
This problem recently came up on #dri-devel again. Adding Alyssa and
Sven who are facing a similar challenge on their work on Apple M1 (if I
understood correctly). Also adding dri-devel for visibility since this
is a very common problem for display in particular.
On M1 the situation is slightly more complicated: the firmware will
allocate a couple of buffers (including the framebuffer) in high memory
(> 4 GiB) and use the IOMMU to map that into an IOVA region below 4 GiB
so that the display hardware can access it. This makes it impossible to
bypass the IOMMU like we do on other chips (in particular to work around
the fault-by-default policy of the ARM SMMU driver). It also means that
in addition to the simple reserved regions I mentioned we need for audio
use-cases and identity mapping use-cases we need for display on Tegra,
we now also need to be able to convey physical to IOVA mappings.
Fitting the latter into the original proposal sounds difficult. A quick
fix would've been to generate a mapping table in memory and pass that to
the kernel using a reserved-memory node (similar to what's done for
example on Tegra for the EMC frequency table on Tegra210) and mark it as
such using a special flag. But that then involves two layers of parsing,
which seems a bit suboptimal. Another way to shoehorn that into the
original proposal would've been to add flags for physical and virtual
address regions and use pairs to pass them using special flags. Again,
this is a bit wonky because it needs these to be carefully parsed and
matched up.
Another downside is that we now have a situation where some of these
regions are no longer "reserved-memory regions" in the traditional
sense. This would require an additional flag in the reserved-memory
region nodes to prevent the IOVA regions from being reserved. By the
way, this is something that would also be needed for the audio use-case
I mentioned before, because the physical memory at that address can
still be used by an operating system.
A more general solution would be to draw a bit from Dmitry's proposal
and introduce a new top-level "iov-reserved-memory" node. This could be
modelled on the existing reserved-memory node, except that the physical
memory pages for regions represented by child nodes would not be marked
as reserved. Only the IOVA range described by the region would be
reserved subsequently by the IOMMU framework and/or IOMMU driver.
The simplest case where we just want to reserve some IOVA region could
then be done like this:
iov-reserved-memory {
    	/*
    	 * Probably safest to default to <2>, <2> here given
    	 * that most IOMMUs support either > 32 bits of IAS
    	 * or OAS.
    	 */
    	#address-cells = <2>;
    	#size-cells = <2>;
firmware: firmware@80000000 {
    		reg = <0 0x80000000 0 0x01000000>;
    	};
    };
audio@30000000 {
    	...
    	iov-memory-regions = <&firmware>;
    	...
    };
Mappings could be represented by an IOV reserved region taking a
reference to the reserved-region that they map:
reserved-memory {
    	#address-cells = <2>;
    	#size-cells = <2>;
/* 16 MiB of framebuffer at top-of-memory */
    	framebuffer: framebuffer@1,ff000000 {
    		reg = <0x1 0xff000000 0 0x01000000>;
    		no-map;
    	};
    };
iov-reserved-memory {
    	/* IOMMU supports only 32-bit output address space */
    	#address-cells = <1>;
    	#size-cells = <1>;
/* 16 MiB of framebuffer mapped to top of IOVA */
    	fb: fb@ff000000 {
    		reg = <0 0xff000000 0 0x01000000>;
    		memory-region = <&framebuffer>;
    	};
    };
display@40000000 {
    	...
    	/* optional? */
    	memory-region = <&framebuffer>;
    	iov-memory-regions = <&fb>;
    	...
    };
It's interesting how identity mapped regions now become a trivial
special case of mappings. All that is needed is to make the reg property
of the IOV reserved region correspond to the reg property of the normal
reserved region. Alternatively, as a small optimization for lazy people
like me, we could just allow these cases to omit the reg property and
instead inherit it from the referenced reserved region.
As the second example shows it might be convenient if memory-region
could be derived from iov-memory-regions. This could be useful for cases
where the driver wants to do something with the physical pages of the
reserved region (such as mapping them and copying out the framebuffer
data to another buffer so that the reserved memory can be recycled). If
we have the IOV reserved region, we could provide an API to extract the
physical reserved region (if it exists). That way we could avoid
referencing it twice in DT. Then again, there's something elegant about
the explicit second reference to. It indicates the intent that we may
want to use the region for something other than just the IOV mapping.
Anyway, this has been long enough. Let me know what you think. Alyssa,
Sven, it'd be interesting to hear if you think this could work as a
solution to the problem on M1.
Rob, I think you might like this alternative because it basically gets
rid of all the points in the original proposal that you were concerned
about. Let me know what you think.
Thierry

Re: [PATCH v2 1/5] dt-bindings: reserved-memory: Document memory region specifier

Signed-off-by: Thierry Reding treding@nvidia.com