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: <DBUE2QU2CH9N.3C32KHT9LHMMB@kernel.org>
Date: Tue, 05 Aug 2025 11:33:22 +0200
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Rob Clark" <robin.clark@....qualcomm.com>
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

(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-----------|

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;