Re: [PATCH 1/3] drm/atomic-helpers: remove legacy_cursor_update hacks

From: "Kazlauskas, Nicholas" <nicholas.kazlauskas@amd.com>
To: Daniel Vetter <daniel.vetter@ffwll.ch>,
	DRI Development <dri-devel@lists.freedesktop.org>
Cc: "Daniel Vetter" <daniel.vetter@intel.com>,
	"Intel Graphics Development" <intel-gfx@lists.freedesktop.org>,
	mikita.lipski@amd.com, "Michel Dänzer" <michel@daenzer.net>
Subject: Re: [PATCH 1/3] drm/atomic-helpers: remove legacy_cursor_update hacks
Date: Thu, 22 Oct 2020 09:36:23 -0400	[thread overview]
Message-ID: <86236137-4a3b-69af-1769-8c4ac8603c08@amd.com> (raw)
In-Reply-To: <20201021163242.1458885-1-daniel.vetter@ffwll.ch>

On 2020-10-21 12:32 p.m., Daniel Vetter wrote:
> The stuff never really worked, and leads to lots of fun because it
> out-of-order frees atomic states. Which upsets KASAN, among other
> things.
> 
> For async updates we now have a more solid solution with the
> ->atomic_async_check and ->atomic_async_commit hooks. Support for that
> for msm and vc4 landed. nouveau and i915 have their own commit
> routines, doing something similar.
> 
> For everyone else it's probably better to remove the use-after-free
> bug, and encourage folks to use the async support instead. The
> affected drivers which register a legacy cursor plane and don't either
> use the new async stuff or their own commit routine are: amdgpu,
> atmel, mediatek, qxl, rockchip, sti, sun4i, tegra, virtio, and vmwgfx.
> 
> Inspired by an amdgpu bug report.
> 
> v2: Drop RFC, I think with amdgpu converted over to use
> atomic_async_check/commit done in
> 
> commit 674e78acae0dfb4beb56132e41cbae5b60f7d662
> Author: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
> Date:   Wed Dec 5 14:59:07 2018 -0500
> 
>      drm/amd/display: Add fast path for cursor plane updates
> 
> we don't have any driver anymore where we have userspace expecting
> solid legacy cursor support _and_ they are using the atomic helpers in
> their fully glory. So we can retire this.
> 
> References: https://bugzilla.kernel.org/show_bug.cgi?id=199425
> Cc: mikita.lipski@amd.com
> Cc: Michel Dänzer <michel@daenzer.net>
> Cc: harry.wentland@amd.com
> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

I'm fine with the idea but it looks like we need modification to amdgpu 
to not break anything:

if (state->legacy_cursor_update) {
/* ... */
		state->async_update =
			!drm_atomic_helper_async_check(dev, state);

We only check async update for legacy_cursor_updates here which won't 
cover the atomic path. I think it's safe to drop the check here but that 
should probably be done before or as part of this series.

Regards,
Nicholas Kazlauskas

> ---
>   drivers/gpu/drm/drm_atomic_helper.c | 13 -------------
>   1 file changed, 13 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
> index a7bcb4b4586c..549a31e6042c 100644
> --- a/drivers/gpu/drm/drm_atomic_helper.c
> +++ b/drivers/gpu/drm/drm_atomic_helper.c
> @@ -1481,13 +1481,6 @@ drm_atomic_helper_wait_for_vblanks(struct drm_device *dev,
>   	int i, ret;
>   	unsigned crtc_mask = 0;
>   
> -	 /*
> -	  * Legacy cursor ioctls are completely unsynced, and userspace
> -	  * relies on that (by doing tons of cursor updates).
> -	  */
> -	if (old_state->legacy_cursor_update)
> -		return;
> -
>   	for_each_oldnew_crtc_in_state(old_state, crtc, old_crtc_state, new_crtc_state, i) {
>   		if (!new_crtc_state->active)
>   			continue;
> @@ -2106,12 +2099,6 @@ int drm_atomic_helper_setup_commit(struct drm_atomic_state *state,
>   			continue;
>   		}
>   
> -		/* Legacy cursor updates are fully unsynced. */
> -		if (state->legacy_cursor_update) {
> -			complete_all(&commit->flip_done);
> -			continue;
> -		}
> -
>   		if (!new_crtc_state->event) {
>   			commit->event = kzalloc(sizeof(*commit->event),
>   						GFP_KERNEL);
> 

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