linux-kernel - Re: [PATCH 29/29] drm/bridge: ti-sn65dsi86: Use bridge

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Message-ID: <20250204-vehement-curious-eagle-b87d7e@houat>
Date: Tue, 4 Feb 2025 09:23:08 +0100
From: Maxime Ripard <mripard@...nel.org>
To: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, 
	Thomas Zimmermann <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, 
	Simona Vetter <simona@...ll.ch>, Andrzej Hajda <andrzej.hajda@...el.com>, 
	Neil Armstrong <neil.armstrong@...aro.org>, Robert Foss <rfoss@...nel.org>, 
	Laurent Pinchart <Laurent.pinchart@...asonboard.com>, Jonas Karlman <jonas@...boo.se>, 
	Jernej Skrabec <jernej.skrabec@...il.com>, Douglas Anderson <dianders@...omium.org>, 
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 29/29] drm/bridge: ti-sn65dsi86: Use bridge_state crtc
 pointer

On Mon, Feb 03, 2025 at 07:49:05PM +0200, Dmitry Baryshkov wrote:
> On Mon, Feb 03, 2025 at 11:01:28AM +0100, Maxime Ripard wrote:
> > Hi Dmitry,
> > 
> > On Thu, Jan 16, 2025 at 03:08:00AM +0200, Dmitry Baryshkov wrote:
> > > On Wed, Jan 15, 2025 at 10:05:36PM +0100, Maxime Ripard wrote:
> > > > The TI sn65dsi86 driver follows the drm_encoder->crtc pointer that is
> > > > deprecated and shouldn't be used by atomic drivers.
> > > > 
> > > > This was due to the fact that we did't have any other alternative to
> > > > retrieve the CRTC pointer. Fortunately, the crtc pointer is now provided
> > > > in the bridge state, so we can move to atomic callbacks and drop that
> > > > deprecated pointer usage.
> > > > 
> > > > Signed-off-by: Maxime Ripard <mripard@...nel.org>
> > > > ---
> > > >  drivers/gpu/drm/bridge/ti-sn65dsi86.c | 41 ++++++++++++++++++++---------------
> > > >  1 file changed, 24 insertions(+), 17 deletions(-)
> > > > 
> > > > diff --git a/drivers/gpu/drm/bridge/ti-sn65dsi86.c b/drivers/gpu/drm/bridge/ti-sn65dsi86.c
> > > > index b3d617505dda7d22b38c000fb79de46376adf3f1..c17d9486cf5c36d61eb00af2bdf9ba1b6f890ffd 100644
> > > > --- a/drivers/gpu/drm/bridge/ti-sn65dsi86.c
> > > > +++ b/drivers/gpu/drm/bridge/ti-sn65dsi86.c
> > > > @@ -242,15 +242,16 @@ static void ti_sn65dsi86_write_u16(struct ti_sn65dsi86 *pdata,
> > > >  	u8 buf[2] = { val & 0xff, val >> 8 };
> > > >  
> > > >  	regmap_bulk_write(pdata->regmap, reg, buf, ARRAY_SIZE(buf));
> > > >  }
> > > >  
> > > > -static u32 ti_sn_bridge_get_dsi_freq(struct ti_sn65dsi86 *pdata)
> > > > +static u32 ti_sn_bridge_get_dsi_freq(struct ti_sn65dsi86 *pdata,
> > > > +				     struct drm_bridge_state *bridge_state)
> > > >  {
> > > >  	u32 bit_rate_khz, clk_freq_khz;
> > > >  	struct drm_display_mode *mode =
> > > > -		&pdata->bridge.encoder->crtc->state->adjusted_mode;
> > > > +		&bridge_state->crtc->state->adjusted_mode;
> > > 
> > > At least we should document why is it safe to follow the crtc->state.
> > 
> > What do you have in mind there? crtc->state is a pointer that is widely
> > used, what is there to document?
> 
> If I understand correctly, crtc->state is safe to be used during atomic
> callbacks only or if the mutex is being held. However this function is
> also being called from ti_sn65dsi86_enable_comms(), which is in turn is
> used in ti_sn65dsi86_resume(). Is it safe? Why?

It's not safe, and it wasn't before this series. I'll send a patch
trying to fix it. However, I can't test it and fixing individual locking
issues isn't really the point of this series either, so I might as well
drop it.

Maxime

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