Re: [PATCH drm-next v5 03/14] drm: manager to keep track of GPUs VA mappings

[Danilo Krummrich]

On 6/22/23 15:54, Christian König wrote:
> Am 20.06.23 um 14:23 schrieb Danilo Krummrich:
> > Hi Christian,
> >
> > On 6/20/23 08:45, Christian König wrote:
> > > Hi Danilo,
> > >
> > > sorry for the delayed reply. I've trying to dig myself out of a hole 
> > > at the moment.
> >
> > No worries, thank you for taking a look anyway!
> >
> > > Am 20.06.23 um 02:42 schrieb Danilo Krummrich:
> > > > [SNIP]
> > > > diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
> > > > index bbc721870c13..5ec8148a30ee 100644
> > > > --- a/include/drm/drm_gem.h
> > > > +++ b/include/drm/drm_gem.h
> > > > @@ -36,6 +36,8 @@
> > > >   #include <linux/kref.h>
> > > >   #include <linux/dma-resv.h>
> > > > +#include <linux/list.h>
> > > > +#include <linux/mutex.h>
> > > >   #include <drm/drm_vma_manager.h>
> > > > @@ -379,6 +381,18 @@ struct drm_gem_object {
> > > >        */
> > > >       struct dma_resv _resv;
> > > > +    /**
> > > > +     * @gpuva:
> > > > +     *
> > > > +     * Provides the list of GPU VAs attached to this GEM object.
> > > > +     *
> > > > +     * Drivers should lock list accesses with the GEMs &dma_resv lock
> > > > +     * (&drm_gem_object.resv).
> > > > +     */
> > > > +    struct {
> > > > +        struct list_head list;
> > > > +    } gpuva;
> > > > +
> > > >       /**
> > > >        * @funcs:
> > > >        *
> > >
> > > I'm pretty sure that it's not a good idea to attach this directly to 
> > > the GEM object.
> >
> > Why do you think so? IMHO having a common way to connect mappings to 
> > their backing buffers is a good thing, since every driver needs this 
> > connection anyway.
> >
> > E.g. when a BO gets evicted, drivers can just iterate the list of 
> > mappings and, as the circumstances require, invalidate the 
> > corresponding mappings or to unmap all existing mappings of a given 
> > buffer.
> >
> > What would be the advantage to let every driver implement a driver 
> > specific way of keeping this connection?
>
> Flexibility. For example on amdgpu the mappings of a BO are groups by VM 
> address spaces.
>
> E.g. the BO points to multiple bo_vm structures which in turn have lists 
> of their mappings.

Isn't this (almost) the same relationship I introduce with the GPUVA 
manager?

If you would switch over to the GPUVA manager right now, it would be 
that every GEM has a list of it's mappings (the gpuva list). The mapping 
is represented by struct drm_gpuva (of course embedded in driver 
specific structure(s)) which has a pointer to the VM address space it is 
part of, namely the GPUVA manager instance. And the GPUVA manager keeps 
a maple tree of it's mappings as well.

If you still would like to *directly* (indirectly you already have that 
relationship) keep a list of GPUVA managers (VM address spaces) per GEM, 
you could still do that in a driver specific way.

Do I miss something?

> Additional to that there is a state maschine associated with the 
> mappings, e.g. if the page tables are up to date or need to be updated 
> etc....
>
> > Do you see cases where this kind of connection between mappings and 
> > backing buffers wouldn't be good enough? If so, which cases do you 
> > have in mind? Maybe we can cover them in a common way as well?
>
> Yeah, we have tons of cases like that. But I have no idea how to 
> generalize them.

They could still remain to be driver specific then, right?

> > > As you wrote in the commit message it's highly driver specific what 
> > > to map and where to map it.
> >
> > In the end the common case should be that in a VA space at least every 
> > mapping being backed by a BO is represented by a struct drm_gpuva.
>
> Oh, no! We also have mappings not backed by a BO at all! For example for 
> partial resident textures or data routing to internal hw etc...
>
> You can't be sure that a mapping is backed by a BO at all.

I fully agree, that's why I wrote "the common case should be that in a 
VA space at least every mapping *being backed by a BO* is represented by 
a struct drm_gpuva".

Mappings not being backed by an actual BO would not be linked to a GEM 
of course.

> > > Instead I suggest to have a separate structure for mappings in a VA 
> > > space which driver can then add to their GEM objects or whatever they 
> > > want to map into their VMs.
> >
> > Which kind of separate structure for mappings? Another one analogous 
> > to struct drm_gpuva?
>
> Well similar to what amdgpu uses BO -> one structure for each 
> combination of BO and VM -> mappings inside that VM

As explained above, I think that's exactly what the GPUVA manager does, 
just in another order:

BO has list of mappings, mappings have pointer to VM, VM has list (or 
actually a maple tree) of mappings.

You see any advantages or disadvantages of either order of 
relationships? For me it looks like it doesn't really matter which one 
to pick.

- Danilo

> Regards,
> Christian.
>
> > - Danilo
> >
> > > Regards,
> > > Christian.