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Date: Sun, 02 Jun 2024 00:35:36 -0300
From: Val Packett <val@packett.cool>
Subject: Re: [PATCH v3 1/2] drm/rockchip: vop: clear DMA stop bit upon vblank
 on RK3066
To: Heiko =?iso-8859-1?q?St=FCbner?= <heiko@sntech.de>
Cc: stable@vger.kernel.org, Sandy Huang <hjc@rock-chips.com>, Andy Yan
	<andy.yan@rock-chips.com>, Maarten Lankhorst
	<maarten.lankhorst@linux.intel.com>, Maxime Ripard <mripard@kernel.org>,
	Thomas Zimmermann <tzimmermann@suse.de>, David Airlie <airlied@gmail.com>,
	Daniel Vetter <daniel@ffwll.ch>, dri-devel@lists.freedesktop.org,
	linux-arm-kernel@lists.infradead.org, linux-rockchip@lists.infradead.org,
	linux-kernel@vger.kernel.org
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On Mon, May 27 2024 at 20:11:49 -03:00:00, Val Packett=20
<val@packett.cool> wrote:
> The RK3066 VOP sets a dma_stop bit when it's done scanning out a frame
> and needs the driver to acknowledge that by clearing the bit.
>=20
> So unless we clear it "between" frames, the RGB output only shows=20
> noise
> instead of the picture. vblank seems to be the most appropriate place=20
> to
> do it, since it indicates exactly that: that the hardware is done
> with the frame.
>=20
> This seems to be a redundant synchronization mechanism that was=20
> removed
> in later iterations of the VOP hardware block.
>=20
> Fixes: f4a6de8 ("drm: rockchip: vop: add rk3066 vop definitions")
> Cc: stable@vger.kernel.org
> Signed-off-by: Val Packett <val@packett.cool>
> ---
>  drivers/gpu/drm/rockchip/rockchip_drm_vop.c | 6 ++++++
>  drivers/gpu/drm/rockchip/rockchip_drm_vop.h | 1 +
>  drivers/gpu/drm/rockchip/rockchip_vop_reg.c | 1 +
>  3 files changed, 8 insertions(+)
>=20
> diff --git a/drivers/gpu/drm/rockchip/rockchip_drm_vop.c=20
> b/drivers/gpu/drm/rockchip/rockchip_drm_vop.c
> index a13473b2d..2731fe2b2 100644
> --- a/drivers/gpu/drm/rockchip/rockchip_drm_vop.c
> +++ b/drivers/gpu/drm/rockchip/rockchip_drm_vop.c
> @@ -1766,6 +1766,12 @@ static void vop_handle_vblank(struct vop *vop)
>  	}
>  	spin_unlock(&drm->event_lock);
>=20
> +	if (VOP_HAS_REG(vop, common, dma_stop)) {
> +		spin_lock(&vop->reg_lock);
> +		VOP_REG_SET(vop, common, dma_stop, 0);
> +		spin_unlock(&vop->reg_lock);
> +	}
> +

Oops=85 so doing it here actually causes deadlocks, unless we also=20
change all other reg_lock usages to be spin_lock_irq/spin_unlock_irq.

Not sure if doing that or going back to v1 would be better.