On Wed, Oct 07, 2020 at 02:42:26PM -0700, Matthias Kaehlcke wrote: > On Wed, Oct 07, 2020 at 04:17:32PM -0400, Alan Stern wrote: > > The peering relation goes both ways, so it should be included in the > > hub_2_0 description too. Given that, the driver could check hub_2_0's > > peer's DT description for the appropriate resources. > > That mitigates the issue somewhat, however we still have to convince Rob that > both references are needed. Strictly speaking, the peering relation applies to ports, not devices. The representation in DT doesn't have to be symmetrical; as long as the kernel understands it, the kernel can set up its own internal symmetrical respresentation. > > > All this mess can be avoided by having a single instance in control of the > > > resources which is guaranteed to suspend after the USB devices. > > > > Yes. At the cost of registering, adding a driver for, and making users > > aware of a fictitious platform device. > > Registration is trivial and the driver code will be needed anyway, I'm > pretty convinced that a separate platform driver will be simpler than > plumbing things into the hub driver, with the additional checks of who is > suspended or not, etc. If other resources like resets are involved there > could be further possible race conditions at probe time. Another issue is > the sysfs attribute. We said to attach it to the primary hub. What happens > when the primary hub goes away? I guess we could force unbinding the peers > as we did in the driver under discussion to avoid confusion/inconsistencies, > but it's another tradeoff. > > My view of the pros and cons of extending the hub driver vs. having a platform > driver: > > - pros > - sysfs attribute is attached to a USB hub device > - no need to register a platform device (trivial) > - potentially more USB awareness (not clear if needed) > > - cons > - possible races involving resources between peer hubs during initialization > - increased complexity from keeping track of peers, checking suspend order > and avoiding races > - peers are forced to unbind when primary goes away > - need DT links to peers for all USB hubs, not only in the primary > - pollution of the generic hub code with device specific stuff instead > of keeping it in a self contained driver > - sysfs attribute is attached to only one of the hubs, which is better than > having it on both, but not necessarily better than attaching it to the > platform device with the 'control logic' > > So yes, there are tradeoffs, IMO balance isn't as clear as your comment > suggests. Well, I guess I'm okay with either approach. One more thing to keep in mind, though: With the platform device, there should be symlinks from the hubs' sysfs directories to the platform device (and possibly symlinks going the other way as well). Alan Stern
