linux-kernel - Re: [PATCH RESEND 1/2] drm/gpuvm: Send in-place re-maps to the driver as remap

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Message-ID: <CACSVV0270Zk4ApR_pGZ+sKgoi14kvfOo_NvzVq3-QALE0mpFsQ@mail.gmail.com>
Date: Tue, 5 Aug 2025 07:32:45 -0700
From: Rob Clark <rob.clark@....qualcomm.com>
To: Danilo Krummrich <dakr@...nel.org>
Cc: dri-devel@...ts.freedesktop.org, freedreno@...ts.freedesktop.org,
        linux-arm-msm@...r.kernel.org, Danilo Krummrich <dakr@...hat.com>,
        Connor Abbott <cwabbott0@...il.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
        Lyude Paul <lyude@...hat.com>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:DRM DRIVER FOR NVIDIA GEFORCE/QUADRO GPUS" <nouveau@...ts.freedesktop.org>
Subject: Re: [PATCH RESEND 1/2] drm/gpuvm: Send in-place re-maps to the driver
 as remap

On Tue, Aug 5, 2025 at 2:33 AM Danilo Krummrich <dakr@...nel.org> wrote:
>
> (Cc: Thomas, Boris, Matt, Alice)
>
> On Mon Aug 4, 2025 at 11:43 PM CEST, Rob Clark wrote:
> > The 'keep' hint on the unmap is only half useful, without being able to
> > link it to a map cb.  Instead combine the two ops into a remap op to
> > give the driver a chance to figure things out.
> >
> > Signed-off-by: Rob Clark <robin.clark@....qualcomm.com>
> > ---
> >  drivers/gpu/drm/drm_gpuvm.c            | 21 +++++++++++++++++++++
> >  drivers/gpu/drm/nouveau/nouveau_uvmm.c |  3 ++-
> >  2 files changed, 23 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/drm_gpuvm.c b/drivers/gpu/drm/drm_gpuvm.c
> > index bbc7fecb6f4a..e21782a97fbe 100644
> > --- a/drivers/gpu/drm/drm_gpuvm.c
> > +++ b/drivers/gpu/drm/drm_gpuvm.c
> > @@ -2125,6 +2125,27 @@ __drm_gpuvm_sm_map(struct drm_gpuvm *gpuvm,
> >                                offset == req_offset;
> >
> >                       if (end == req_end) {
> > +                             if (merge) {
> > +                                     /*
> > +                                      * This is an exact remap of the existing
> > +                                      * VA (potentially flags change)?  Pass
> > +                                      * this to the driver as a remap so it can
> > +                                      * do an in-place update:
> > +                                      */
> > +                                     struct drm_gpuva_op_map n = {
> > +                                             .va.addr = va->va.addr,
> > +                                             .va.range = va->va.range,
> > +                                             .gem.obj = va->gem.obj,
> > +                                             .gem.offset = va->gem.offset,
> > +                                     };
> > +                                     struct drm_gpuva_op_unmap u = {
> > +                                             .va = va,
> > +                                             .keep = true,
> > +                                     };
> > +
> > +                                     return op_remap_cb(ops, priv, NULL, &n, &u);
> > +                             }
>
> I don't see why this is necessary, a struct drm_gpuva_op_unmap carries the
> struct drm_gpuva to unmap. You can easily compare this to the original request
> you gave to GPUVM, i.e. req_addr, req_range, req_obj, req_offset, etc.
>
> Which is what you have to do for any other unmap operation that has keep == true
> anyways, e.g. if D is the exact same as A, B and C.
>
>         Cur
>         ---
>         1                       N
>         |---A---|---B---|---C---|
>
>         Req
>         ---
>         1                       N
>         |-----------D-----------|

Ugg, this means carrying around more state between the unmap and map
callbacks, vs. just handing all the data to the driver in a single
callback.  For the keep==true case, nouveau just seems to skip the
unmap.. I guess in your case the map operation is tolerant of
overwriting existing mappings so this works out, which isn't the case
with io_pgtable.

I guess I could handle the specific case of an exact in-place remap in
the driver to handle this specific case.  But the example you give
with multiple mappings would be harder to cope with.

I still feel there is some room for improvement in gpuvm to make this
easier for drivers.  Maybe what I proposed isn't the best general
solution, but somehow giving the drivers info about both the unmaps
and maps in the same callback would make things easier (and the remap
callback is _almost_ that).

BR,
-R

>
> In this case you get three unmap ops with keep == true, which you can compare to
> your request to figure out that you can keep the corresponding PTEs.
>
> Besides that it changes the semantics that the documentation mentions and that
> drivers are allowed to rely on, i.e. a struct drm_gpuva_op_remap represents
> an actual change and any call to __drm_gpuvm_sm_map() results in an arbitrary
> number of unmap ops, a maximum of two remap ops and exactly one map operation.
>
> >                               ret = op_unmap_cb(ops, priv, va, merge);
> >                               if (ret)
> >                                       return ret;