LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
From: Alan Stern <firstname.lastname@example.org>
To: Rajat Jain <email@example.com>
Cc: Dmitry Torokhov <firstname.lastname@example.org>,
Rajat Jain <email@example.com>,
Greg Kroah-Hartman <firstname.lastname@example.org>,
Thinh Nguyen <Thinh.Nguyen@synopsys.com>,
Mathias Nyman <email@example.com>,
Andrew Lunn <firstname.lastname@example.org>,
Chris Chiu <email@example.com>,
firstname.lastname@example.org, email@example.com, firstname.lastname@example.org,
Subject: Re: [PATCH 2/2] usb: hub: Mark devices downstream a removable hub, as removable
Date: Wed, 6 Oct 2021 12:08:19 -0400 [thread overview]
Message-ID: <20211006160819.GA659483@rowland.harvard.edu> (raw)
On Tue, Oct 05, 2021 at 04:43:33PM -0700, Rajat Jain wrote:
> On Tue, Oct 5, 2021 at 12:59 PM Alan Stern <email@example.com> 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 <firstname.lastname@example.org> 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 <email@example.com>
> > > 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.
next prev parent reply other threads:[~2021-10-06 16:08 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-29 22:48 [PATCH 1/2] usb: hub: Mark root hubs on removable devices, " Rajat Jain
2021-09-29 22:48 ` [PATCH 2/2] usb: hub: Mark devices downstream a removable hub, " Rajat Jain
2021-09-30 5:31 ` Greg Kroah-Hartman
2021-10-04 22:42 ` Rajat Jain
2021-10-05 14:56 ` Alan Stern
2021-10-05 16:51 ` Dmitry Torokhov
2021-10-05 19:59 ` Alan Stern
2021-10-05 23:43 ` Rajat Jain
2021-10-06 0:41 ` Andrew Lunn
2021-10-06 16:08 ` Alan Stern [this message]
2021-10-06 9:37 ` Oliver Neukum
2021-10-06 16:10 ` Alan Stern
2021-10-06 18:36 ` Oliver Neukum
2021-09-30 5:30 ` [PATCH 1/2] usb: hub: Mark root hubs on removable devices, " Greg Kroah-Hartman
2021-10-04 21:51 ` Rajat Jain
2021-09-30 8:02 ` Oliver Neukum
2021-10-04 21:56 ` Rajat Jain
2021-10-05 11:19 ` Greg Kroah-Hartman
2021-10-05 23:49 ` Rajat Jain
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--subject='Re: [PATCH 2/2] usb: hub: Mark devices downstream a removable hub, as removable' \
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).