linux-kernel - Re: [PATCH v3 1/2] drm/rockchip: vop: clear DMA stop bit upon vblank on RK3066

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Date: Sun, 02 Jun 2024 00:35:36 -0300
From: Val Packett <val@...kett.cool>
To: Heiko Stübner <heiko@...ech.de>
Cc: stable@...r.kernel.org, Sandy Huang <hjc@...k-chips.com>, Andy Yan
	<andy.yan@...k-chips.com>, Maarten Lankhorst
	<maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>,
	Daniel Vetter <daniel@...ll.ch>, dri-devel@...ts.freedesktop.org,
	linux-arm-kernel@...ts.infradead.org, linux-rockchip@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/2] drm/rockchip: vop: clear DMA stop bit upon vblank
 on RK3066



On Mon, May 27 2024 at 20:11:49 -03:00:00, Val Packett 
<val@...kett.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.
> 
> So unless we clear it "between" frames, the RGB output only shows 
> noise
> instead of the picture. vblank seems to be the most appropriate place 
> to
> do it, since it indicates exactly that: that the hardware is done
> with the frame.
> 
> This seems to be a redundant synchronization mechanism that was 
> removed
> in later iterations of the VOP hardware block.
> 
> Fixes: f4a6de8 ("drm: rockchip: vop: add rk3066 vop definitions")
> Cc: stable@...r.kernel.org
> Signed-off-by: Val Packett <val@...kett.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(+)
> 
> diff --git a/drivers/gpu/drm/rockchip/rockchip_drm_vop.c 
> 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);
> 
> +	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… so doing it here actually causes deadlocks, unless we also 
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.