Re: [RFC] Future TTM DMA direction

From: Thomas Hellstrom
Date: Wed Jan 25 2012 - 13:34:01 EST

OK, revisiting this again, please see inline below,


On 01/10/2012 06:46 PM, Jerome Glisse wrote:
On Mon, Jan 09, 2012 at 11:11:06AM +0100, Daniel Vetter wrote:
On Mon, Jan 09, 2012 at 10:37:28AM +0100, Thomas Hellstrom wrote:
Hi!

When TTM was originally written, it was assumed that GPU apertures
could address pages directly, and that the CPU could access those
pages without explicit synchronization. The process of binding a
page to a GPU translation table was a simple one-step operation, and
we needed to worry about fragmentation in the GPU aperture only.

Now that we "sort of" support DMA memory there are three things I
think are missing:

1) We can't gracefully handle coherent DMA OOMs or coherent DMA
(Including CMA) memory fragmentation leading to failed allocations.
2) We can't handle dynamic mapping of pages into and out of dma, and
corresponding IOMMU space shortage or fragmentation, and CPU
synchronization.
3) We have no straightforward way of moving pages between devices.

I think a reasonable way to support this is to make binding to a
non-fixed (system page based) TTM memory type a two-step binding
process, so that a TTM placement consists of (DMA_TYPE, MEMORY_TYPE)
instead of only (MEMORY_TYPE).

In step 1) the bo is bound to a specific DMA type. These could be
for example:
(NONE, DYNAMIC, COHERENT, CMA), .... device dependent types could be
allowed as well.
In this step, we perform dma_sync_for_device, or allocate
dma-specific pages maintaining LRU lists so that if we receive a DMA
memory allocation OOM, we can unbind bo:s bound to the same DMA
type. Standard graphics cards would then, for example, use the NONE
DMA type when run on bare metal or COHERENT when run on Xen. A
"COHERENT" OOM condition would then lead to eviction of another bo.
(Note that DMA eviction might involve data copies and be costly, but
still better than failing).
Binding with the DYNAMIC memory type would mean that CPU accesses
are disallowed, and that user-space CPU page mappings might need to
be killed, with a corresponding sync_for_cpu if they are faulted in
again (perhaps on a page-by-page basis). Any attempt to bo_kmap() a
bo page bound to DYNAMIC DMA mapping should trigger a BUG.

In step 2) The bo is bound to the GPU in the same way it's done
today. Evicting from DMA will of course also trigger an evict from
GPU, but an evict from GPU will not trigger a DMA evict.

Making a bo "anonymous" and thus moveable between devices would then
mean binding it to the "NONE" DMA type.

Comments, suggestions?
Well I think we need to solve outstanding issues in the dma_buf framework
first. Currently dma_buf isn't really up to par to handle coherency
between the cpu and devices and there's also not yet any way to handle dma
address space fragmentation/exhaustion.

I fear that if you jump ahead with improving the ttm support alone we
might end up with something incompatible to the stuff dma_buf eventually
will grow, resulting in decent amounts of wasted efforts.

Cc'ed a bunch of relevant lists to foster input from people.

For a starter you seem to want much more low-level integration with the
dma api than existing users commonly need. E.g. if I understand things
correctly drivers just call dma_alloc_coherent and the platform/board code
then decides whether the device needs a contigious allocation from cma or
whether something else is good, too (e.g. vmalloc for the cpu + iommu).
Another thing is that I think doing lru eviction in case of dma address
space exhaustion (or fragmentation) needs at least awereness of what's
going on in the upper layers. iommus are commonly shared between devices
and I presume that two ttm drivers sitting behind the same iommu and
fighting over it's resources can lead to some hilarious outcomes.

Cheers, Daniel
I am with Daniel here, while i think the ttm API change you propose are
good idea, i think most of the issue you are listing need to be addressed
at lower level. If ttm keeps doing its own things for GPU in its own little
area we gonna endup in a dma getto ;)

dma space exhaustion is somethings that is highly platform specific, on
x86 platform it's very unlikely to happen for AMD, Intel or NVidia GPU.
While on the ARM platform it's more likely situation, at least on current
generation.

OK. You and Daniel have convinced me to leave OOM- and fragmentation handling
out of TTM, but I still think TTM DMA placement might be a good thing to look at
when time allows.


I believe that the dma api to allocate memory are just too limited for the
kind of device and support we are having. The association to a device is
too restrictive. I would rather see some new API to allocate DMA/IOMMU,
something more flexible and more in line with the dma-buf work.

I believe all dma allocation have a set of restriction. dma mask of the
device, is there an iommu or not, iommu dma mask if any, iommu has a limited
address space (note that recent x86 iommu don't have such limit). In the
end it's not only the device dma mask that matter but also the iommu.
For space exhaustion core dma/iommu need to grow reclaim callback so each
driver that use the dma/iommu space can try to free/unbind things.

In the end what i would really like to see is all the ttm page allocation
helper moved to core kernel helper, and getting rid of ttm memory accounting
by properly hooking up into core memory accouting so that core kernel
infrastructure have a clue about how much memory each process really use
include memory use by device like GPU unlike today.

I agree it would be possible and desirable to do that for the movable pages backing bo data,
however for non-movable data structures (buffer object metadata, fence objects) etc.
there still needs to be a way to avoid exhausting kmalloced memory.


Anyway my point is that the 3 point you want to address need to be addressed
first at core common DMA code rather then at ttm level. This would benefit
all user of dma-buf and also other devices.


agreed.

/Thomas



Cheers,
Jerome
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