[RFC] Future TTM DMA direction

[Jerome Glisse]

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.

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.

To achieve this i believe we need new/improved core DMA api, growing it
with dma-buf make more sense to me. Somethings like :
sg_table dma_alloc(pid, flags|physmask, size, reclaim_callback, reclaim_data)
dma_bind, dma_unbind.

The bind/unbind is to allow to just the unbinding from the iommu if any
without deallocating the pages. We could make the reclaim callback get some
flags to tell if only bind/unbind is enough or if it needs to free pages too.

The reclaim_callback might be a more common things for instance we could
also do something more like dma_pool.
dma_unit_create(name, iommu, reclaim_callback, reclaim_data)
dma_unit_alloc(unit, pid, size, mask, alignment)

So one could create an unit for each iommu there is and assuming device
behind or capable of using the iommu can then share it.

I am pretty sure all this can be in common code shared accross different
arch, thought maybe the iommu API might needs some additions.

Note that here i am assuming that alloc can fail due to memory limit on
the process on behalf of which we are allocating. But then it's up to
upper level to properly deals with allocation failure.

For allocation that are kernel only we could use some special process id
or kernel id.

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.

Cheers,
Jerome