Re: [PATCH 2/2] drm/bridge: dw-hdmi: Always add the bridge in the global bridge list

[Liu Ying <victor.liu@nxp.com>]

Hi Laurent,

On Sun, 2020-06-28 at 11:22 +0300, Laurent Pinchart wrote:
> Hi Liu,
> 
> (CC'ing Sam)
> 
> Thank you for the patch.

Thanks for your review.

> 
> On Tue, Jun 16, 2020 at 05:04:52PM +0800, Liu Ying wrote:
> > It doesn't hurt to add the bridge in the global bridge list also
> > for
> > platform specific dw-hdmi drivers which are based on the component
> > framework.  This can be achieved by moving the drm_bridge_add()
> > function
> > call from dw_hdmi_probe() to __dw_hdmi_probe().  Moreover, putting
> > the
> > drm_bridge_add() function call prior to the interrupt registration
> > and
> > enablement ensures that the mutex hpd_mutex embedded in the
> > structure
> > drm_bridge can be initialized in drm_bridge_add() beforehand, which
> > avoids accessing the uninitialized mutex in case people want to
> > call
> > function drm_bridge_hpd_notify() with the mutex locked internally
> > to
> > handle hot plug detection event in the interrupt handler
> > dw_hdmi_irq().
> > 
> > Cc: Andrzej Hajda <a.hajda@samsung.com>
> > Cc: Neil Armstrong <narmstrong@baylibre.com>
> > Cc: Laurent Pinchart <Laurent.pinchart@ideasonboard.com>
> > Cc: Jonas Karlman <jonas@kwiboo.se>
> > Cc: Jernej Skrabec <jernej.skrabec@siol.net>
> > Cc: David Airlie <airlied@linux.ie>
> > Cc: Daniel Vetter <daniel@ffwll.ch>
> > Cc: Boris Brezillon <boris.brezillon@collabora.com>
> > Cc: Jerome Brunet <jbrunet@baylibre.com>
> > Cc: Cheng-Yi Chiang <cychiang@chromium.org>
> > Cc: Dariusz Marcinkiewicz <darekm@google.com>
> > Cc: Archit Taneja <architt@codeaurora.org>
> > Cc: Jose Abreu <joabreu@synopsys.com>
> > Cc: dri-devel@lists.freedesktop.org
> > Cc: NXP Linux Team <linux-imx@nxp.com>
> > Signed-off-by: Liu Ying <victor.liu@nxp.com>
> > ---
> > Laurent,
> > 
> > I may see the uninitialized mutex accessing issue with
> > i.MX dw-hdmi after applying your below patch set[1].
> > I think patch '[22/27] drm: bridge: dw-hdmi: Make connector
> > creation optional'
> > triggers the issue.
> > 
> > [1] 
> > https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fpatchwork.kernel.org%2Fcover%2F11569709%2F&amp;data=02%7C01%7Cvictor.liu%40nxp.com%7Cca86b38a5fbc49a44b1c08d81b3c5cde%7C686ea1d3bc2b4c6fa92cd99c5c301635%7C0%7C0%7C637289293354715359&amp;sdata=C7kz8HONVSNMYkQGb4h9uVcdZHqJVSmtwgnN4J2cKws%3D&amp;reserved=0
> > 
> >  drivers/gpu/drm/bridge/synopsys/dw-hdmi.c | 34 ++++++++++++++-----
> > ------------
> >  1 file changed, 15 insertions(+), 19 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
> > b/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
> > index da84a91..4711700 100644
> > --- a/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
> > +++ b/drivers/gpu/drm/bridge/synopsys/dw-hdmi.c
> > @@ -3247,17 +3247,25 @@ __dw_hdmi_probe(struct platform_device
> > *pdev,
> >  
> >  	dw_hdmi_init_hw(hdmi);
> >  
> > +	hdmi->bridge.driver_private = hdmi;
> > +	hdmi->bridge.funcs = &dw_hdmi_bridge_funcs;
> > +#ifdef CONFIG_OF
> > +	hdmi->bridge.of_node = pdev->dev.of_node;
> > +#endif
> > +
> > +	drm_bridge_add(&hdmi->bridge);
> 
> This would introduce a race condition where a display driver could
> get
> hold of the bridge before it is fully initialized.

Yes, it seems it's a bit too early to add the bridge in the global
bridge list.

> 
> I fear the right fix for this may be to add a drm_bridge_init()
> function
> to move mutex initialization away from drm_bridge_add(). That's a
> rather
> intrusive change I'm afraid :-(

Looking into the issue more closely, it may be solved by moving
drm_bridge_add() from dw_hdmi_probe() to __dw_hdmi_probe() just before
__dw_hdmi_probe() returns successfully and a counterpart movement for
drm_bridge_remove(). The key is that hdmi->bridge.dev must be !NULL
when drm_bridge_hpd_notify() is called in dw_hdmi_irq() and
hdmi->bridge.dev is set in drm_bridge_attach() after drm_bridge_add()
is called.

This looks more safe because there is no logic change as
dw_hdmi_probe()/dw_hdmi_remove() see and just an additional
drm_bridge_add()/drm_bridge_remove() call as
dw_hdmi_bind()/dw_hdmi_unbind() see.

I plan to test this with i.MX dw-hdmi tomorrow.

> 
> > +
> >  	irq = platform_get_irq(pdev, 0);
> >  	if (irq < 0) {
> >  		ret = irq;
> > -		goto err_iahb;
> > +		goto err_irq;
> >  	}
> >  
> >  	ret = devm_request_threaded_irq(dev, irq, dw_hdmi_hardirq,
> >  					dw_hdmi_irq, IRQF_SHARED,
> >  					dev_name(dev), hdmi);
> >  	if (ret)
> > -		goto err_iahb;
> > +		goto err_irq;
> >  
> >  	/*
> >  	 * To prevent overflows in HDMI_IH_FC_STAT2, set the clk
> > regenerator
> > @@ -3290,12 +3298,6 @@ __dw_hdmi_probe(struct platform_device
> > *pdev,
> >  			hdmi->ddc = NULL;
> >  	}
> >  
> > -	hdmi->bridge.driver_private = hdmi;
> > -	hdmi->bridge.funcs = &dw_hdmi_bridge_funcs;
> > -#ifdef CONFIG_OF
> > -	hdmi->bridge.of_node = pdev->dev.of_node;
> > -#endif
> > -
> >  	if (hdmi->version >= 0x200a)
> >  		hdmi->connector.ycbcr_420_allowed =
> >  			hdmi->plat_data->ycbcr_420_allowed;
> > @@ -3357,6 +3359,8 @@ __dw_hdmi_probe(struct platform_device *pdev,
> >  
> >  	return hdmi;
> >  
> > +err_irq:
> > +	drm_bridge_remove(&hdmi->bridge);
> >  err_iahb:
> >  	clk_disable_unprepare(hdmi->iahb_clk);
> >  	if (hdmi->cec_clk)
> > @@ -3371,6 +3375,8 @@ __dw_hdmi_probe(struct platform_device *pdev,
> >  
> >  static void __dw_hdmi_remove(struct dw_hdmi *hdmi)
> >  {
> > +	drm_bridge_remove(&hdmi->bridge);
> > +
> >  	if (hdmi->audio && !IS_ERR(hdmi->audio))
> >  		platform_device_unregister(hdmi->audio);
> >  	if (!IS_ERR(hdmi->cec))
> > @@ -3396,22 +3402,12 @@ static void __dw_hdmi_remove(struct dw_hdmi
> > *hdmi)
> >  struct dw_hdmi *dw_hdmi_probe(struct platform_device *pdev,
> >  			      const struct dw_hdmi_plat_data
> > *plat_data)
> >  {
> > -	struct dw_hdmi *hdmi;
> > -
> > -	hdmi = __dw_hdmi_probe(pdev, plat_data);
> > -	if (IS_ERR(hdmi))
> > -		return hdmi;
> > -
> > -	drm_bridge_add(&hdmi->bridge);
> > -
> > -	return hdmi;
> > +	return __dw_hdmi_probe(pdev, plat_data);
> >  }
> >  EXPORT_SYMBOL_GPL(dw_hdmi_probe);
> 
> Do we need to keep __dw_hdmi_probe() and dw_hdmi_probe(), can't we
> rename __dw_hdmi_probe() to dw_hdmi_probe() ? Same for the remove
> functions.

Yes, the renaming makes sense. Will do that in V2 if the above new
solution stands.

Regards,
Liu Ying

> 
> >  
> >  void dw_hdmi_remove(struct dw_hdmi *hdmi)
> >  {
> > -	drm_bridge_remove(&hdmi->bridge);
> > -
> >  	__dw_hdmi_remove(hdmi);
> >  }
> >  EXPORT_SYMBOL_GPL(dw_hdmi_remove);
> 
> 

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