On Tue, Oct 05, 2021 at 04:43:33PM -0700, Rajat Jain wrote: > Hello, > > On Tue, Oct 5, 2021 at 12:59 PM Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> 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. > > I have no preference either way, and can do that if that is more acceptable. > > > > > 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. > > However I do think it makes sense to have the bus control whether this > attribute applies to it or not. The sysfs ABI quoted by Dmitry above is a little vague. It seems to say that only certain buses can determine whether a device is removable, but this simply isn't true, because any device downstream from something removable will itself be removable, no matter what kind of bus it's on. > Determining the first point in a > hierarchy of devices, where a device can be removed is highly bus > specific (set_usb_port_removable()). Yes, the bus must be at least partially responsible for _determining_ the value of the removable attribute. But the attribute should _apply_ to all devices, regardless of what bus they are on. To be more precise, the bus can determine whether a device's upstream link (the first link in the chain leading from the device back to the CPU) can be hot-unplugged. The device is removable if any of the links in that chain are hot-unpluggable. > AFAIK, the primary reason / use of this attribute was to distinguish > devices that can be removed by the user, and really all such devices > (at least the ones that matter to user) today sit either on PCI or USB > bus. We intend to use this attribute to segregate internal devices > from external devices, and collect some statistics about usb device > usage this way. There is also a VM case that I think Dmitry or Benson > on this thread can elaborate more about. There seem to be hundreds of > other bus types and I'm not sure if we want to unnecessarily flood the > sysfs with a removable attribute under each device. sysfs already contains a lot of mostly unused information. I don't think adding one or two more will hurt much. Alan Stern
