linux-kernel - Re: [PATCH v2 3/3] gpuvm: remove gem.gpuva.lock_dep

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Message-Id: <DC8VTZ5OZY42.1OCS3QJHZGPAK@kernel.org>
Date: Fri, 22 Aug 2025 12:25:34 +0200
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Alice Ryhl" <aliceryhl@...gle.com>
Cc: "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>, "Boris
 Brezillon" <boris.brezillon@...labora.com>, "Daniel Almeida"
 <daniel.almeida@...labora.com>, "Steven Price" <steven.price@....com>,
 "Liviu Dudau" <liviu.dudau@....com>, "Rob Clark"
 <robin.clark@....qualcomm.com>, "Rob Herring" <robh@...nel.org>, "Miguel
 Ojeda" <ojeda@...nel.org>, "Boqun Feng" <boqun.feng@...il.com>, "Gary Guo"
 <gary@...yguo.net>, Björn Roy Baron
 <bjorn3_gh@...tonmail.com>, "Benno Lossin" <lossin@...nel.org>, "Andreas
 Hindborg" <a.hindborg@...nel.org>, "Trevor Gross" <tmgross@...ch.edu>,
 <dri-devel@...ts.freedesktop.org>, <linux-kernel@...r.kernel.org>,
 <rust-for-linux@...r.kernel.org>
Subject: Re: [PATCH v2 3/3] gpuvm: remove gem.gpuva.lock_dep_map

On Fri Aug 22, 2025 at 11:28 AM CEST, Alice Ryhl wrote:
> diff --git a/include/drm/drm_gpuvm.h b/include/drm/drm_gpuvm.h
> index 4a22b9d848f7b3d5710ca554f5b01556abf95985..598ba376b9430cdd4ab7bacdc15de031a887cf71 100644
> --- a/include/drm/drm_gpuvm.h
> +++ b/include/drm/drm_gpuvm.h
> @@ -196,10 +196,20 @@ enum drm_gpuvm_flags {
>  	 */
>  	DRM_GPUVM_RESV_PROTECTED = BIT(0),
>  
> +	/**
> +	 * @DRM_GPUVM_IMMEDIATE_MODE: use the locking scheme that makes it safe
> +	 * to modify the GPUVM during the fence signalling path

I think this isn't entirely true yet or at least can be ambiguous for now,
because it doesn't prevent users from having DRM_GPUVM_RESV_PROTECTED set, which
requires the DMA resv lock to destroy a struct drm_gpuvm_bo, which may happen
from drm_gpuva_unlink().

So, for now, until we have the deferred drop idea in place, it also
requires DRM_GPUVM_RESV_PROTECTED to not be set.

(Eventually, we of course want both to be represented as a single flag, such
that it becomes an either or.)

I also wouldn't say "makes it safe to", but rather "makes it possible to
safely". We have no control over what the users do with the mutex. :)

NIT: missing period

> +	 *
> +	 * When set, gpuva.lock is used to protect gpuva.list in all GEM
> +	 * objects associated with this GPUVM. Otherwise, the GEMs dma-resv is
> +	 * used.
> +	 */
> +	DRM_GPUVM_IMMEDIATE_MODE = BIT(1),
> +
>  	/**
>  	 * @DRM_GPUVM_USERBITS: user defined bits
>  	 */
> -	DRM_GPUVM_USERBITS = BIT(1),
> +	DRM_GPUVM_USERBITS = BIT(2),
>  };