On Tue, Jan 08, 2019 at 03:33:54PM +0100, Andrzej Hajda wrote: > On 08.01.2019 14:21, Russell King - ARM Linux 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. > > > Supplier usually is registered in it's probe time, so there is short > period of time when supplier is available, but the probe is not yet > finished - quite unsafe, but not impossible, especially if there exists > some kind of notifications about resource appearance (MIPI-DSI case). At some point during the supplier probe, the resource becomes available to consumers. At that point, device links can be setup before the supplier has finished probing. So any driver that provides resources to another driver will, at some point during the provider's probe, make resources available, and therefore be a candidate for device links to be created _before_ the probe function has returned. What is a problem is if the provider publishes resources, and then fails its probe function, causing the resource to disappear. Taking DRM bridge, drm_bridge_add() returns void - it never fails. Most bridge drivers do drm_bridge_add() as the very last step before returning zero from their probe function. There are a few exceptions. megachips-stdpxxxx-ge-b850v3-fw.c is already buggy - it calls devm_request_threaded_irq(), which if it fails, the bridge is left added to the global list of bridges. Since this structure was allocated with devm_kzalloc(), it will be freed when the probe function fails. So, any failure here (eg a deferred probe because the IRQ controller is not available) already creates a latent bug just waiting to bite. tc358764.c is another case where the bridge is published prior to initialisation completion, but it looks like that's under the control of the "host" (consumer) driver. mtk_hdmi.c enables clocks for audio after registering the bridge, which may fail, but are unlikely to. However, moving them prior to drm_bridge_add() probably won't hurt and avoids the "published interfaces which then vanish" problem. Then we have exynos_drm_mic.c and tda998x_drv.c, but I think the latter's error path after drm_bridge_add() can be eliminated if we transitioned to device links instead of the component helper. Outside DRM, take regulators - at some point during a regulator supplier's probe function, the resource will be published, and as soon as it is, it's available for regulator_get() to find it and setup a device link before the probe function has finished. >From what I can tell, in the situation where a supplier makes resources available, the consumer binds to the resource, and then the supplier goes away, the device link will remain, and the consumer will not be unbound, which leads to an unexpected state. The solution to this is the age old kernel rule of "don't publish your interfaces until you've completed initialisation". IOW, publish at a point where you aren't going to fail. -- RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up According to speedtest.net: 11.9Mbps down 500kbps up _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
