linux-kernel - Re: [PATCH] drm/msm: Replace custom dumb_map

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Message-ID: <cv4we57zxltpys6qf43nxldwwogjwxyglp2hj3ld6talvlteqp@dsen5g3v24t7>
Date: Mon, 15 Dec 2025 22:48:30 +0200
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Swaraj Gaikwad <swarajgaikwad1925@...il.com>
Cc: Rob Clark <robin.clark@....qualcomm.com>,
        Dmitry Baryshkov <lumag@...nel.org>,
        Abhinav Kumar <abhinav.kumar@...ux.dev>,
        Jessica Zhang <jesszhan0024@...il.com>, Sean Paul <sean@...rly.run>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
        "open list:DRM DRIVER for Qualcomm display hardware" <linux-arm-msm@...r.kernel.org>,
        "open list:DRM DRIVER for Qualcomm display hardware" <dri-devel@...ts.freedesktop.org>,
        "open list:DRM DRIVER for Qualcomm display hardware" <freedreno@...ts.freedesktop.org>,
        open list <linux-kernel@...r.kernel.org>, skhan@...uxfoundation.org,
        david.hunter.linux@...il.com
Subject: Re: [PATCH] drm/msm: Replace custom dumb_map_offset with generic
 helper

On Mon, Dec 15, 2025 at 02:28:50AM +0000, Swaraj Gaikwad wrote:
> The msm driver implements a custom dumb_map_offset callback. This
> implementation acquires the msm_gem_lock, but the underlying
> drm_gem_create_mmap_offset() function is already thread-safe regarding
> the VMA offset manager (it acquires the mgr->vm_lock internally).

Other pieces are using msm_gem_lock() / msm_gem_unlock(), which
translates to dma_resv_lock() / dma_resv_unlock(), so you need to
describe, why it's still safe wrt. that code.

> 
> Switching to the generic drm_gem_dumb_map_offset() helper provides
> several benefits:
> 1. Removes the unnecessary locking overhead (locking leftovers).
> 2. Adds a missing check to reject mapping of imported objects, which is
>    invalid for dumb buffers.
> 3. Allows for the removal of the msm_gem_dumb_map_offset() wrapper and
>    the msm_gem_mmap_offset() helper function.
> 
> The logic from msm_gem_mmap_offset() has been inlined into
> msm_ioctl_gem_info() to maintain functionality without the separate
> helper.
> 
> This addresses the TODO:
> "Documentation/gpu/todo.rst: Remove custom dumb_map_offset implementations"
> 
> Signed-off-by: Swaraj Gaikwad <swarajgaikwad1925@...il.com>
> ---
> 
>  Compile-tested only.
> 

-- 
With best wishes
Dmitry