On Fri, Jun 03, 2022 at 06:52:05AM -0700, Doug Anderson wrote: > On Fri, Jun 3, 2022 at 3:19 AM Dmitry Baryshkov > <dmitry.baryshkov@xxxxxxxxxx> wrote: > > On Fri, 3 Jun 2022 at 11:21, Maxime Ripard <maxime@xxxxxxxxxx> wrote: > > > > > > On Tue, May 31, 2022 at 02:06:34PM -0700, Doug Anderson wrote: > > > > On Mon, May 23, 2022 at 10:00 AM Doug Anderson <dianders@xxxxxxxxxxxx> wrote: > > > > > On Sat, May 21, 2022 at 2:17 AM Maxime Ripard <maxime@xxxxxxxxxx> wrote: > > > > > > On Tue, May 10, 2022 at 12:29:43PM -0700, Douglas Anderson wrote: > > > > > > > This adds a devm managed version of drm_bridge_add(). Like other > > > > > > > "devm" function listed in drm_bridge.h, this function takes an > > > > > > > explicit "dev" to use for the lifetime management. A few notes: > > > > > > > * In general we have a "struct device" for bridges that makes a good > > > > > > > candidate for where the lifetime matches exactly what we want. > > > > > > > * The "bridge->dev->dev" device appears to be the encoder > > > > > > > device. That's not the right device to use for lifetime management. > > > > > > > > > > > > > > Suggested-by: Dmitry Baryshkov <dmitry.baryshkov@xxxxxxxxxx> > > > > > > > Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx> > > > > > > > > > > > > If we are to introduce more managed helpers, I think it'd be wiser to > > > > > > introduce them as DRM-managed, and not device managed. > > > > > > > > > > > > Otherwise, you'll end up in a weird state when a device has been removed > > > > > > but the DRM device is still around. > > > > > > > > > > I'm kinda confused. In this case there is no DRM device for the bridge > > > > > and, as per my CL description, "bridge-dev->dev" appears to be the > > > > > encoder device. I wasn't personally involved in discussions about it, > > > > > but I was under the impression that this was expected / normal. Thus > > > > > we can't make this DRM-managed. > > > > > > > > Since I didn't hear a reply, > > > > > > Gah, I replied but it looks like somehow it never reached the ML... > > > > > > Here was my original reply: > > > > > > > > > This adds a devm managed version of drm_bridge_add(). Like other > > > > > > "devm" function listed in drm_bridge.h, this function takes an > > > > > > explicit "dev" to use for the lifetime management. A few notes: > > > > > > * In general we have a "struct device" for bridges that makes a good > > > > > > candidate for where the lifetime matches exactly what we want. > > > > > > * The "bridge->dev->dev" device appears to be the encoder > > > > > > device. That's not the right device to use for lifetime management. > > > > > > > > > > > > Suggested-by: Dmitry Baryshkov <dmitry.baryshkov@xxxxxxxxxx> > > > > > > Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx> > > > > > > > > > > If we are to introduce more managed helpers, I think it'd be wiser to > > > > > introduce them as DRM-managed, and not device managed. > > > > > > > > > > Otherwise, you'll end up in a weird state when a device has been removed > > > > > but the DRM device is still around. > > > >=20 > > > > I'm kinda confused. In this case there is no DRM device for the bridge > > > > and, as per my CL description, "bridge-dev->dev" appears to be the > > > > encoder device. > > > > > > bridge->dev seems right though? > > > > > > > I wasn't personally involved in discussions about it, but I was under > > > > the impression that this was expected / normal. Thus we can't make > > > > this DRM-managed. > > > > > > Still, I don't think devm is the right solution to this either. > > > > > > The underlying issue is two-fold: > > > > > > - Encoders can have a pointer to a bridge through of_drm_find_bridge > > > or similar. However, bridges are traditionally tied to their device > > > lifetime (by calling drm_bridge_add in probe, and drm_bridge_remove > > > in remove). Encoders will typically be tied to the DRM device > > > however, and that one sticks around until the last application > > > closes it. We can thus very easily end up with a dangling pointer, > > > and a use-after-free. > > > > > > - It's not the case yet, but it doesn't seem far fetch to expose > > > properties of bridges to the userspace. In that case, the userspace > > > would be likely to still hold references to objects that aren't > > > there anymore when the bridge is gone. > > > > > > The first is obviously a larger concern, but if we can find a solution > > > that would accomodate the second it would be great. > > > > > > As far as I can see, we should fix in two steps: > > > > > > - in drm_bridge_attach, we should add a device-managed call that will > > > unregister the main DRM device. We don't allow to probe the main DRM > > > device when the bridge isn't there yet in most case, so it makes > > > sense to remove it once the bridge is no longer there as well. > > > > The problem is that I do not see a good way to unregister the main DRM > > device outside of it's driver code. > > > > > > > > - When the DRM device is removed, have the core cleanup any bridge > > > registered. That will remove the need to have drm_bridge_remove in > > > the first place. > > > > > > > I'll assume that my response addressed your concerns. Assuming I get > > > > reviews for the other two patches in this series I'll plan to land > > > > this with Dmitry's review. > > > > > > I still don't think it's a good idea to merge it. It gives an illusion > > > of being safe, but it's really far from it. > > > > It is more of removing the boilerplate code spread over all the > > drivers rather than about particular safety. > > > > I'd propose to land devm_drm_bridge_add (and deprecate calling > > drm_bridge_remove from the bridge driver at some point) and work on > > the whole drm_device <-> drm_bridge problem in the meantime. > > At this point it has been landed in drm-misc-next as per my response > to the cover letter. If need be we can revert it and rework the ps8640 > driver to stop using it but it wouldn't change the lifetime of the > bridge. I'm not going to rework the bridge lifetime rules here. If > nothing else it seems like having the devm function at least would > make it obvious which drivers need to be fixed whenever the bridge > lifetime problem gets solved. Not really, no. The issue exists whether or not the driver would be using devm. Anyway, what's done is done. Could you please ping earlier than a few minutes before applying the patch next time though? We could have easily prevented that situation. Maxime
