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Date:   Thu, 25 Apr 2019 20:59:15 +0300
From:   Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To:     Matt Redfearn <matt.redfearn@thinci.com>
Cc:     Andrzej Hajda <a.hajda@samsung.com>,
        Philippe Cornu <philippe.cornu@st.com>,
        "dri-devel@lists.freedesktop.org" <dri-devel@lists.freedesktop.org>,
        Matthew Redfearn <matthew.redfearn@thinci.com>,
        Nickey Yang <nickey.yang@rock-chips.com>,
        Heiko Stuebner <heiko@sntech.de>,
        Archit Taneja <architt@codeaurora.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>
Subject: Re: [PATCH] drm/bridge/synopsys: dsi: Don't blindly call post_disable
Message-ID: <20190425175915.GB12024@pendragon.ideasonboard.com>
References: <CGME20190424142210epcas3p403bd5159852ab4e90eca1be67800ecde@epcas3p4.samsung.com>
 <20190424142148.25927-1-matt.redfearn@thinci.com>
 <3da4c0a3-a77b-5128-63f7-9c8d6b012c68@samsung.com>
 <95c36782-7eaf-2224-bb52-cff0a53d98d8@thinci.com>
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Hi Matt,

On Thu, Apr 25, 2019 at 12:39:27PM +0000, Matt Redfearn wrote:
> On 25/04/2019 13:13, Andrzej Hajda wrote:
> > On 24.04.2019 16:22, Matt Redfearn wrote:
> >> The DRM documentation states that post_disable is an optional callback.
> >> As such an implementing device may not populate it. To avoid panicing
> >> the kernel by calling a NULL function pointer, we should NULL check it
> >> before blindy calling it.
> >>
> >> Signed-off-by: Matt Redfearn <matt.redfearn@thinci.com>
> > 
> >> ---
> >>
> >>   drivers/gpu/drm/bridge/synopsys/dw-mipi-dsi.c | 3 ++-
> >>   1 file changed, 2 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/gpu/drm/bridge/synopsys/dw-mipi-dsi.c b/drivers/gpu/drm/bridge/synopsys/dw-mipi-dsi.c
> >> index 38e88071363..0ee440216b8 100644
> >> --- a/drivers/gpu/drm/bridge/synopsys/dw-mipi-dsi.c
> >> +++ b/drivers/gpu/drm/bridge/synopsys/dw-mipi-dsi.c
> >> @@ -805,7 +805,8 @@ static void dw_mipi_dsi_bridge_post_disable(struct drm_bridge *bridge)
> >>   	 * This needs to be fixed in the drm_bridge framework and the API
> >>   	 * needs to be updated to manage our own call chains...
> >>   	 */
> >> -	dsi->panel_bridge->funcs->post_disable(dsi->panel_bridge);
> >> +	if (dsi->panel_bridge->funcs->post_disable)
> >> +		dsi->panel_bridge->funcs->post_disable(dsi->panel_bridge);
> >>   
> > 
> > Why not drm_bridge_post_disable ?
> 
> Ah - that seems like a nicer fix! Do you think the comment above 
> describing why this function pointer is called directly can be removed 
> as well if we go this route?

It shouldn't be necessary to call ->post_disable manually here as the
bridge core handles it internally. This is a hack to work around a
problem, and should be fixed properly.

> If someone calls drm_bridge_post_disable() on the Synposys DSI 
> drm_bridge it will go on to call post_disable on all other bridges in 
> the chain, in addition to us calling them here. Is it an issue to call 
> it multiple times?

It depends on the panel implementation, but in general it's not a good
idea. It may happen to work, but could break at any time in the future.

> >>   	if (dsi->slave) {
> >>   		dw_mipi_dsi_disable(dsi->slave);

-- 
Regards,

Laurent Pinchart