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

From: Heiko Stuebner <heiko@sntech.de>
To: Val Packett <val@packett.cool>
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
Subject: Re: [PATCH v3 1/2] drm/rockchip: vop: clear DMA stop bit upon vblank on RK3066
Date: Sat, 08 Jun 2024 16:49:50 +0200	[thread overview]
Message-ID: <4253237.VLH7GnMWUR@phil> (raw)
In-Reply-To: <CNOFES.QFA1L3UJ3SH82@packett.cool>

Am Sonntag, 2. Juni 2024, 05:35:36 CEST schrieb Val Packett:
> 
> On Mon, May 27 2024 at 20:11:49 -03:00:00, Val Packett 
> <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.
> > 
> > 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@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(+)
> > 
> > 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.

then please go back to v1 (as v4) :-) .

I.e. regular spinlock vs. spin_lock_irq will have performance
implications and this "feature" is a one-time only thing used
only on a more than 10 year old soc, so it really must not affect
stuff coming after it.

Heiko

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