On Tue, Jan 8, 2019 at 2:22 PM Russell King - ARM Linux <linux@xxxxxxxxxxxxxxx> wrote: > On Tue, Jan 08, 2019 at 01:27:56PM +0100, Andrzej Hajda wrote: > > On 08.01.2019 12:38, Russell King - ARM Linux wrote: > > > On Tue, Jan 08, 2019 at 12:25:34PM +0100, Andrzej Hajda wrote: > > >> Issues with device links have nothing to do with hotplugging, they are > > >> generic - lifetime of the objects (drm_bridge, drm_panel) is just > > >> slightly different of lifetime of device links, and this is racy even if > > >> you do not want hotplugging. Moreover since drm_dev is not device (has > > >> no associated struct device) assuming we can reuse its parent to create > > >> device link results in circular dependencies. > > > How about having the device links created depending on whether the > > > main drm driver wants them or not - that would mean that Exynos > > > could continue avoiding them, but others that want them can have > > > the links? > > > > > > That should be OK for Exynos. But regardless of Exynos device_links at > > the current state will not work correctly with bridges/panels as I > > described earlier. > > However, I'm not sure you're correct with your interpretation of the > documentation. Firstly, the documentation says: > > Another example for an inconsistent state would be a device link that > represents a driver presence dependency, yet is added from the consumer's > ->probe callback while the supplier hasn't probed yet: Had the driver core > known about the device link earlier, it wouldn't have probed the consumer > in the first place. The onus is thus on the consumer to check presence of > the supplier after adding the link, and defer probing on non-presence. > > This is fine - if we add the device link from of_drm_find_bridge(), we > will be in the consumer's ->probe function, and the supplier must have > been probed for us to find the struct drm_bridge. > > Secondly, device links are created by the regulator support whenever a > regulator is "got" - between the supplier and the consumer. I'm not > sure that regulator uses this in a safe way since it looks to me like > there could be a race between the point where the regulator has been > found and the point that the device link is created, and the regulator > supplier being unbound. Regulator uses stateless device links to > order PM, but not to remove consumers when the supplier goes away. > > Finally, I believe that CCF is looking at using device links as well, > which will mean a link is established by a clk_get() type operation > and released in a clk_put() operation. There hasn't been any code > merged for this, but I have seen it discussed. > > These all have in common one thing - a device link is created at the > point that the resource is obtained and removed when the resource is > released. > > I don't see how DRM bridges are any different from any other resource > in the system, and why you think that device links wouldn't work for > DRM bridges. Yeah, for PM ordering device links should work for bridges and panels. I think all the questions/confusion is whether they also work for load/unload ordering, which they atm don't anyway because if you unload a provider and then reload it, the consumer isn't added to the reprobe list (unlikely what component achieves, which does call master bind again in this case). I also think the entire discussion around whether you can hotadd bridges/panels after drm_dev_register is orthogonal, or at least should be. If you want to support hot-adding, they neither can you use device links nor component, because they both create a more static connection between producer and consumer. Aside from hotadding anything aside from drm_connector isn't really supported, and you need to roll your own locking and everything (which exynos really doesn't seem to do - the fact it works if you're careful doesn't really change that). -Daniel -- Daniel Vetter Software Engineer, Intel Corporation +41 (0) 79 365 57 48 - http://blog.ffwll.ch _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
