On 05.10.21 21:59, Alan Stern wrote: > On Tue, Oct 05, 2021 at 09:51:02AM -0700, Dmitry Torokhov wrote: >> Hi Alan, >> >> On Tue, Oct 5, 2021 at 7:56 AM Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: >>> As I understand it, the "removable" property refers specifically to >>> the device's upstream link, not to whether _any_ of the links leading >>> from the device to the computer could be removed. >> No, that is not what it means. I'll cite our sysfs ABI: >> >> What: /sys/devices/.../removable >> Date: May 2021 >> Contact: Rajat Jain <rajatxjain@xxxxxxxxx> >> Description: >> Information about whether a given device can be removed from the >> platform by the user. This is determined by its subsystem in a >> bus / platform-specific way. This attribute is only present for >> devices that can support determining such information: >> >> "removable": device can be removed from the platform by the user >> "fixed": device is fixed to the platform / cannot be removed >> by the user. >> "unknown": The information is unavailable / cannot be deduced. >> >> Currently this is only supported by USB (which infers the >> information from a combination of hub descriptor bits and >> platform-specific data such as ACPI) and PCI (which gets this >> from ACPI / device tree). >> >> It specifically talks about _platform_, not about properties of some >> peripheral attached to a system. Note that the wording is very similar >> to what we had for USB devices that originally implemented "removable" >> attribute: > In that case, shouldn't Rajat's patch change go into the driver core > rather than the hub driver? _Every_ device downstream from a > removable link should count as removable, yes? Not just the USB > devices. In theory yes. If your HC is removable by that logic every device is. That renders the information content of 'removable' to zero. Everything is removable. > And to say that the attribute is supported only by USB and PCI is > misleading, since it applies to every device downstream from a > removable link. Exactly and it is a difference. If you know that a device is removable you must not disable hotplug detection on that port if you want full functionality. While if you know that a device is not removable you may straight up cut power, even if the _parent_ is still removable. The device tree is a tree and if you want to know whether hotplugging is possible (let's ignore hibernation), you need to walk the tree top to bottom. Regards Oliver
