Re: [PATCH] drm: Rip out totally bogus vga_switcheroo->can_switch locking

From: Thierry Reding <thierry.reding@gmail.com>
To: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Thierry Reding <treding@nvidia.com>,
	DRI Development <dri-devel@lists.freedesktop.org>
Subject: Re: [PATCH] drm: Rip out totally bogus vga_switcheroo->can_switch locking
Date: Wed, 23 Apr 2014 09:28:55 +0200	[thread overview]
Message-ID: <20140423072853.GD31226@ulmo> (raw)
In-Reply-To: <1398199519-21821-1-git-send-email-daniel.vetter@ffwll.ch>

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On Tue, Apr 22, 2014 at 10:45:19PM +0200, Daniel Vetter wrote:
> So I just wanted to add a new field to struct drm_device and
> accidentally stumbled over something. According to comments
> dev->open_count is protected by dev->count_lock, but that's totally
> not the case. It's protected by drm_global_mutex.
> 
> Unfortunately the vga switcheroo callbacks took this comment at face
> value. The problem is that we can't just take the drm_global_mutex
> because:
> - It would lead to a locking inversion with the driver load/unload
>   paths.
> - It wouldn't actually protect anything, for that we'd need to wrap
>   the entire vga switcheroo code in the drm_global_mutex. And I'm not
>   sure whether that would actually solve anything.
> 
> What we probably want is a try_to_grab_switcheroo reference kind of
> thing which is used in the driver's ->open callback. Then we could
> move all that ->can_switch madness into the vga switcheroo core where
> it really belongs.
> 
> But since that would amount to real work take the easy way out and
> just add a comment. It's definitely not going to make anything worse
> since doing switcheroo state changes while restarting X just isn't
> recommended. Even though the delayed switching code does exactly that.
> 
> v2:
> - Simplify the ->can_switch implementations more (Thierry)
> - Fix comment about the dev->open_count locking (Thierry)
> 
> Cc: Thierry Reding <treding@nvidia.com>
> Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com> (v1)
> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> ---
>  drivers/gpu/drm/i915/i915_dma.c        | 11 ++++++-----
>  drivers/gpu/drm/nouveau/nouveau_vga.c  | 11 ++++++-----
>  drivers/gpu/drm/radeon/radeon_device.c | 11 ++++++-----
>  include/drm/drmP.h                     |  2 +-
>  4 files changed, 19 insertions(+), 16 deletions(-)

Looks good:

Reviewed-by: Thierry Reding <treding@nvidia.com>

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