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

[Christian König]

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.

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.

> > 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.

> > 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

Regards,
Christian.

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