linux-kernel - Re: [PATCH 2/2] usb: hub: Mark devices downstream a removable hub, as removable

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Date:   Wed, 6 Oct 2021 11:37:58 +0200
From:   Oliver Neukum <oneukum@...e.com>
To:     Alan Stern <stern@...land.harvard.edu>,
        Dmitry Torokhov <dtor@...gle.com>
CC:     Rajat Jain <rajatja@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thinh Nguyen <Thinh.Nguyen@...opsys.com>,
        Mathias Nyman <mathias.nyman@...ux.intel.com>,
        Andrew Lunn <andrew@...n.ch>,
        Chris Chiu <chris.chiu@...onical.com>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        levinale@...gle.com, bleung@...gle.com, rajatxjain@...il.com,
        jsbarnes@...gle.com, pmalani@...gle.com
Subject: Re: [PATCH 2/2] usb: hub: Mark devices downstream a removable hub, as
 removable


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@...land.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@...il.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.
> 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