On 9/14/21 4:07 PM, Daniel Vetter wrote:
On Tue, Sep 14, 2021 at 12:38:00PM +0200, Thomas Hellström wrote:
On Tue, 2021-09-14 at 10:53 +0200, Christian König wrote:
Am 14.09.21 um 10:27 schrieb Thomas Hellström:
On Tue, 2021-09-14 at 09:40 +0200, Christian König wrote:
Am 13.09.21 um 14:41 schrieb Thomas Hellström:
[SNIP]
Let's say you have a struct ttm_object_vram and a struct
ttm_object_gtt, both subclassing drm_gem_object. Then I'd
say
a
driver would want to subclass those to attach identical
data,
extend functionality and provide a single i915_gem_object
to
the
rest of the driver, which couldn't care less whether it's
vram or
gtt? Wouldn't you say having separate struct
ttm_object_vram
and a
struct ttm_object_gtt in this case would be awkward?. We
*want* to
allow common handling.
Yeah, but that's a bad idea. This is like diamond
inheritance
in C++.
When you need the same functionality in different backends
you
implement that as separate object and then add a parent
class.
It's the exact same situation here. With struct
ttm_resource
you
let *different* implementation flavours subclass it,
which
makes it
awkward for the driver to extend the functionality in a
common way
by subclassing, unless the driver only uses a single
implementation.
Well the driver should use separate implementations for
their
different domains as much as possible.
Hmm, Now you lost me a bit. Are you saying that the way we do
dynamic
backends in the struct ttm_buffer_object to facilitate driver
subclassing is a bad idea or that the RFC with backpointer is
a
bad
idea?
Or if you mean diamond inheritance is bad, yes that's basically
my
point.
That diamond inheritance is a bad idea. What I don't understand
is
why
you need that in the first place?
Information that you attach to a resource are specific to the
domain
where the resource is allocated from. So why do you want to
attach
the
same information to a resources from different domains?
Again, for the same reason that we do that with struct
i915_gem_objects
and struct ttm_tts, to extend the functionality. I mean information
that we attach when we subclass a struct ttm_buffer_object doesn't
necessarily care about whether it's a VRAM or a GTT object. In
exactly
the same way, information that we want to attach to a struct
ttm_resource doesn't necessarily care whether it's a system or a
VRAM
resource, and need not be specific to any of those.
In this particular case, as memory management becomes asynchronous,
you
can't attach things like sg-tables and gpu binding information to
the
gem object anymore, because the object may have a number of
migrations
in the pipeline. Such things need to be attached to the structure
that
abstracts the memory allocation, and which may have a completely
different lifetime than the object itself.
In our particular case we want to attach information for cached
page
lookup and and sg-table, and moving forward probably the gpu
binding
(vma) information, and that is the same information for any
ttm_resource regardless where it's allocated from.
Typical example: A pipelined GPU operation happening before an
async
eviction goes wrong. We need to error capture and reset. But if we
look
at the object for error capturing, it's already updated pointing to
an
after-eviction resource, and the resource sits on a ghost object
(or in
the future when ghost objects go away perhaps in limbo somewhere).
We need to capture the memory pointed to by the struct ttm_resource
the
GPU was referencing, and to be able to do that we need to cache
driver-
specific info on the resource. Typically an sg-list and GPU binding
information.
Anyway, that cached information needs to be destroyed together with
the
resource and thus we need to be able to access that information
from
the resource in some way, regardless whether it's a pointer or
whether
we embed the struct resource.
I think it's pretty important here that we (using the inheritance
diagram below) recognize the need for D to inherit from A, just
like we
do for objects or ttm_tts.
Looking at
https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FMultiple_inheritance%23%2Fmedia%2FFile%3ADiamond_inheritance.svg&data=04%7C01%7Cchristian.koenig%40amd.com%7C268bb562db8548b285b408d977598b2c%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637672048739103176%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=bPyDqiSF%2FHFZbl74ux0vfwh3uma5hZIUf2xbzb9yZz8%3D&reserved=0
1)
A would be the struct ttm_resource itself,
D would be struct i915_resource,
B would be struct ttm_range_mgr_node,
C would be struct i915_ttm_buddy_resource
And we need to resolve the ambiguity using the awkward union
construct, iff we need to derive from both B and C.
Struct ttm_buffer_object and struct ttm_tt instead have B) and
C)
being dynamic backends of A) or a single type derived from A)
Hence
the problem doesn't exist for these types.
So the question from last email remains, if ditching this RFC,
can
we
have B) and C) implemented by helpers that can be used from D)
and
that don't derive from A?
Well we already have that in the form of drm_mm. I mean the
ttm_range_manager is just a relatively small glue code which
implements
the TTMs resource interface using the drm_mm object and a
spinlock.
IIRC
that less than 200 lines of code.
So you should already have the necessary helpers and just need to
implement the resource manager as far as I can see.
I mean I reused the ttm_range_manager_node in for amdgpu_gtt_mgr
and
could potentially reuse a bit more of the ttm_range_manager code.
But
I
don't see that as much of an issue, the extra functionality there
is
just minimal.
Sure but that would give up the prereq of having reusable resource
manager implementations. What happens if someone would like to
reuse
the buddy manager? And to complicate things even more, the
information
we attach to VRAM resources also needs to be attached to system
resources. Sure we could probably re-implement a combined system-
buddy-
range manager, but that seems like something overly complex.
The other object examples resolve the diamond inheritance with a
pointer to the specialization (BC) and let D derive from A.
TTM resources do it backwards. If we can just recognize that and
ponder
what's the easiest way to resolve this given the current design, I
actually think we'd arrive at a backpointer to allow downcasting
from A
to D.
Yeah, but I think you are approaching that from the wrong side.
For use cases like this I think you should probably have the
following
objects and inheritances:
1. Driver specific objects like i915_sg, i915_vma which don't inherit
anything from TTM.
2. i915_vram_node which inherits from ttm_resource or a potential
ttm_buddy_allocator.
3. i915_gtt_node which inherits from ttm_range_manger_node.
4. Maybe i915_sys_node which inherits from ttm_resource as well.
The managers for the individual domains then provide the glue code to
implement both the TTM resource interface as well as a driver
specific
interface to access the driver objects.
Well yes, but this is not really much better than the union thing. More
memory efficient but also more duplicated type definitions and manager
definitions and in addition overriding the default system resource
manager, not counting the kerneldoc needed to explain why all this is
necessary.
It was this complexity I was trying to get away from in the first
place.
I honestly don't think the union thing is the worst. At least as long as
we're reworking i915 at a fairly invasive pace it's probably the lest
worst approach.
For the specific case of sg list I'm also not sure how great our current
i915 design of "everything is an sg" really is. In the wider community
there's clear rejection of sg for p2p addresses, so having this as a
per-ttm_res_manager kind of situation is probably not the worst.
OK well, I'm no defender of the usage of sg list itself, but I was under
the impression that as long as it was either only visible to the driver
code itself or constructed using dma_map_resource() returned addresses
for p2p it would be OK?
In that world every ttm_res_manager would have it's own implementation of
binding into ptes, which then iterate over the pagetables with some common
abstraction. So in a way more of a helper approach for the i915
implementations of the various hooks, at the cost of a bit of code
duplication.
I do agree with Christian that the various backpointers to sort out the
diamond inheritence issue isn't not great. The other options aren't pretty
either, but at least it's more contained to i915.
OK, I guess I will have to implement whatever ends up prettiest without
the back pointer then. I wonder whether there is something we can think
of in the future to avoid these diamond- or diamond like inheritances.
/Thomas
-Daniel
/Thomas
Amdgpu just uses a switch/case for now, but you could as well extend
the
ttm_resource_manager_func table and upcast that inside the driver.
Regards,
Christian.
Thanks,
Thomas
Regards,
Christian.
Thanks,
Thomas
