[RFC PATCH 0/2] Support for TI Page-based Address Translator

[Andrew F. Davis]

Hello all,

So I've got a new IP on our new SoC I'm looking to make use of and would
like some help figuring out what framework best matches its function. The
IP is called a "Page-based Address Translator" or PAT. A PAT instance
(there are 5 of these things on our J721e device[0]) is basically a
really simple IOMMU sitting on the interconnect between the device bus
and what is effectively our northbridge called
MSMC (DRAM/SRAM/L3-Cache/Coherency controller).

Simplified it looks about like this:

         CPUs
          |
DRAM --- MSMC --- SRAM/L3
          |
        NAVSS - (PATs)
          |
  --- Device Bus ---------
 |      |      |          |
Device  Device  Device   etc..

Each PAT has a set a window in high memory (about 0x48_0000_0000 area)
for which any transaction with an address targeting its window will be
routed into that PAT. The PAT then does a simple calculation based on
the how far into the window the address is and the current page size,
does a lookup to an internally held table of translations, then sends the
transaction back out on the interconnect with a new address. Usually this
address should be towards somewhere in DRAM, but can be some other device
or even back into PAT (I'm not sure there is a valid use-case for this
but just a point of interest).

My gut reaction is that this is an IOMMU which belongs in the IOMMU
subsystem. But there are a couple oddities that make me less sure it is
really suitable for the IOMMU framework. First it doesn't sit in front of
any devices, it sits in front of *all* devices, this means we would have
every device claim it as an IOMMU parent, even though many devices also
have a traditional IOMMU connected. Second, there is only a limited
window of address space per PAT, this means we will get fragmentation and
allocation failures on occasion, in this way it looks to me more like AGP
GART. Third, the window is in high-memory, so unlike some IOMMU devices
which can be used to allow DMA to high-mem from low-mem only devices, PAT
can't be used for that. Lastly it doesn't provide any isolation, if the
access does not target the PAT window it is not used (that is not to say
we don't have isolation, just that it is taken care of by other parts of
the interconnect).

This means, to me, that PAT has one main purpose: making
physically-contiguous views of scattered pages in system memory for DMA.
But it does that really well, the whole translation table is held in a
PAT-internal SRAM giving 1 bus cycle latency and at full bus bandwidth.

So what are my options here, is IOMMU the right way to go or not?

Looking around the kernel I also see the char dev ARP/GART interface
which looks like a good fit, but also looks quite dated and my guess
deprecated at this point. Moving right along..

Another thing I saw is we have the support upstream of the DMM device[1]
available in some OMAPx/AM57x SoCs. I'm a little more familiar with this
device. The DMM is a bundle of IPs and in fact one of them is called
"PAT" and it even does basically the same thing this incarnation of "PAT"
does. It's upstream integration design is a bit questionable
unfortunately, the DMM support was integrated into the OMAPDRM display
driver, which does make some sense then given its support for rotation
(using TILER IP contained in DMM). The issue with this was that the
DMM/TILER/PAT IP was not part of the our display IP, but instead out at
the end of the shared device bus, inside the external memory controller.
Like this new PAT this meant that any IP that could make use of it, but
only the display framework could actually provide buffers backed by it.
This meant, for instance, if we wanted to decode some video buffer using
our video decoder we would have to allocate from DRM framework then pass
that over to the V4L2 system. This doesn't make much sense and required
the user-space to know about this odd situation and allocate from the
right spot or else have to use up valuable CMA space or waste memory with
dedicated carveouts.

Another idea would be to have this as a special central allocator
(exposed through DMA-BUF heaps[2] or ION) that would give out normal
system memory as a DMA-BUF but remap it with PAT if a device that only
supports contiguous memory tries to attach/map that DMA-BUF.

One last option would be to allow user-space to choose to make the buffer
contiguous when it needs. That's what the driver in this series allows.
We expose a remapping device, user-space passes it a non-contiguous
DMA-BUF handle and it passes a contiguous one back. Simple as that.

So how do we use this, lets take Android for example, we don't know at
allocation time if a rendering buffer will end up going back into the GPU
for further processing, or if it will be consumed directly by the display.
This is a problem for us as our GPU can work with non-contiguous buffers
but our display cannot, so any buffers that could possibly go to the
display at some point currently needs to be allocated as contiguous from
the start, this leads to a lot of unneeded use of carveout/CMA memory.
With this driver on the other hand, we allocate regular non-contiguous
system memory (again using DMA-BUF heaps, but ION could work here too),
then only when a buffer is about to be sent to the display we pass the
handle to this DMA-BUF to our driver here and take the handle it gives
back and pass that to the display instead.

As said, it is probably not the ideal solution but it does work and was
used for some early testing of the IP.

Well, sorry for the wall of text.
Any and all suggestions very welcome and appreciated.

Thanks,
Andrew

[0] http://www.ti.com/lit/pdf/spruil1
[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/gpu/drm/omapdrm/omap_dmm_tiler.c
[2] https://lkml.org/lkml/2019/6/6/1211

Andrew F. Davis (2):
  dt-bindings: soc: ti: Add TI PAT bindings
  soc: ti: Add Support for the TI Page-based Address Translator (PAT)

 .../devicetree/bindings/misc/ti,pat.txt       |  34 ++
 drivers/soc/ti/Kconfig                        |   9 +
 drivers/soc/ti/Makefile                       |   1 +
 drivers/soc/ti/ti-pat.c                       | 569 ++++++++++++++++++
 include/uapi/linux/ti-pat.h                   |  44 ++
 5 files changed, 657 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/misc/ti,pat.txt
 create mode 100644 drivers/soc/ti/ti-pat.c
 create mode 100644 include/uapi/linux/ti-pat.h

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