Am 11.09.21 um 08:07 schrieb Thomas Hellström:
On Fri, 2021-09-10 at 19:03 +0200, Christian König wrote:
Am 10.09.21 um 17:30 schrieb Thomas Hellström:
On Fri, 2021-09-10 at 16:40 +0200, Christian König wrote:
Am 10.09.21 um 15:15 schrieb Thomas Hellström:
Both the provider (resource manager) and the consumer (the TTM
driver)
want to subclass struct ttm_resource. Since this is left for
the
resource
manager, we need to provide a private pointer for the TTM
driver.
Provide a struct ttm_resource_private for the driver to
subclass
for
data with the same lifetime as the struct ttm_resource: In the
i915
case
it will, for example, be an sg-table and radix tree into the
LMEM
/VRAM pages that currently are awkwardly attached to the GEM
object.
Provide an ops structure for associated ops (Which is only
destroy() ATM)
It might seem pointless to provide a separate ops structure,
but
Linus
has previously made it clear that that's the norm.
After careful audit one could perhaps also on a per-driver
basis
replace the delete_mem_notify() TTM driver callback with the
above
destroy function.
Well this is a really big NAK to this approach.
If you need to attach some additional information to the resource
then
implement your own resource manager like everybody else does.
Well this was the long discussion we had back then when the
resource
mangagers started to derive from struct resource and I was under
the
impression that we had come to an agreement about the different
use-
cases here, and this was my main concern.
Ok, then we somehow didn't understood each other.
I mean, it's a pretty big layer violation to do that for this use-
case.
Well exactly that's the point. TTM should not have a layer design in
the
first place.
Devices, BOs, resources etc.. are base classes which should implement
a
base functionality which is then extended by the drivers to implement
the driver specific functionality.
That is a component based approach, and not layered at all.
The TTM resource manager doesn't want to know about this data at
all,
it's private to the ttm resource user layer and the resource
manager
works perfectly well without it. (I assume the other drivers that
implement their own resource managers need the data that the
subclassing provides?)
Yes, that's exactly why we have the subclassing.
The fundamental problem here is that there are two layers wanting
to
subclass struct ttm_resource. That means one layer gets to do that,
the
second gets to use a private pointer, (which in turn can provide
yet
another private pointer to a potential third layer). With your
suggestion, the second layer instead is forced to subclass each
subclassed instance it uses from the first layer provides?
Well completely drop the layer approach/thinking here.
The resource is an object with a base class. The base class
implements
the interface TTM needs to handle the object, e.g.
create/destroy/debug
etc...
Then we need to subclass this object because without any additional
information the object is pretty pointless.
One possibility for this is to use the range manager to implement
something drm_mm based. BTW: We should probably rename that to
something
like ttm_res_drm_mm or similar.
Sure I'm all in on that, but my point is this becomes pretty awkward
because the reusable code already subclasses struct ttm_resource. Let
me give you an example:
Prereqs:
1) We want to be able to re-use resource manager implementations among
drivers.
2) A driver might want to re-use multiple implementations and have
identical data "struct i915_data" attached to both
Well that's the point I don't really understand. Why would a driver want
to do this?
It's perfectly possible that you have ttm_range_manager extended and a
potential ttm_page_manager, but that are two different objects then
which also need different handling.
....
This would be identical to how we subclass a struct ttm_buffer_object
or a struct ttm_tt. But It can't look like this because then we can't
reuse exising implementations that *already subclass* struct
ttm_resource.
What we have currently ttm_resource-wise is like having a struct
tt_bo_vram, a struct ttm_bo_system, a struct ttm_bo_gtt and trying to
subclass them all combined into a struct i915_bo. It would become
awkward without a dynamic backend that facilitates subclassing a single
struct ttm_buffer_object?
Why? They all implement different handling.
When you add a private point to ttm_resource you allow common handling
which doesn't take into account that this ttm_resource object is
subclassed.
So basically the question boils down to: Why do we do struct
ttm_resources differently?
ttm_buffer_object is a subclass of drm_gem_object and I hope to make
ttm_device a subclass of drm_device in the near term.
I really try to understand what you mean hear, but I even after reading
that multiple times I absolutely don't get it.
Regards,
Christian.
What we should avoid is to abuse TTM resource interfaces in the
driver,
e.g. what i915 is currently doing. This is a TTM->resource mgr
interface
and should not be used by drivers at all.
Yes I guess that can be easily fixed when whatever we end up with above
lands.
Ofc we can do that, but it does indeed feel pretty awkward.
In any case, if you still think that's the approach we should go
for,
I'd need to add init() and fini() members to the
ttm_range_manager_func
struct to allow subclassing without having to unnecessarily copy
the
full code?
Yes, exporting the ttm_range_manager functions as needed is one thing
I
wanted to do for the amdgpu_gtt_mgr.c code as well.
Just don't extend the function table but rather directly export the
necessary functions.
Sure.
/Thomas
