Re: [PATCH] drm/bridge/synopsys: dsi: Don't blindly call post_disable

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

On Thu, Apr 25, 2019 at 08:59:15PM +0300, Laurent Pinchart wrote:
> 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.

Double-checking the driver, the .attach() operation doesn't propagate to
the next bridge, so the bridge core will not know about it, and will not
propagate .post_disable() either. I think this should be fixed in a way
that uses the drm bridge core infrastructure.

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

-- 
Regards,

Laurent Pinchart