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From: Alan Stern <stern@rowland.harvard.edu>
To: Oliver Neukum <oneukum@suse.com>
Cc: Dmitry Torokhov <dtor@google.com>,
	Rajat Jain <rajatja@google.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Thinh Nguyen <Thinh.Nguyen@synopsys.com>,
	Mathias Nyman <mathias.nyman@linux.intel.com>,
	Andrew Lunn <andrew@lunn.ch>,
	Chris Chiu <chris.chiu@canonical.com>,
	linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org,
	levinale@google.com, bleung@google.com, rajatxjain@gmail.com,
	jsbarnes@google.com, pmalani@google.com
Subject: Re: [PATCH 2/2] usb: hub: Mark devices downstream a removable hub, as removable
Date: Wed, 6 Oct 2021 12:10:58 -0400	[thread overview]
Message-ID: <20211006161058.GB659483@rowland.harvard.edu> (raw)
In-Reply-To: <c219168c-2727-0b44-7a05-7c0e31770de4@suse.com>

On Wed, Oct 06, 2021 at 11:37:58AM +0200, Oliver Neukum wrote:
> 
> 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@rowland.harvard.edu> 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@gmail.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.
> 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.

So we should add a new attribute.  Call it "unpluggable", perhaps.  It 
will say whether the device's immediate upstream link is 
hot-unpluggable.  Then the device is removable if its parent is 
removable or if it is unpluggable.

> > 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.

Adding the "unpluggable" attribute should take care of this, right?

Alan Stern

  reply	other threads:[~2021-10-06 16:11 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
2021-10-06  9:37             ` Oliver Neukum
2021-10-06 16:10               ` Alan Stern [this message]
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

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