ABI: sysfs-devices-removable: make a table valid as ReST markup

Changeset 70f400d4d957 ("driver core: Move the "removable" attribute from USB to core")
added a table to the file, but not using a valid ReST format.

Change it to avoid build issues when producing documentation with
"make htmldocs".

Fixes: 70f400d4d957 ("driver core: Move the "removable" attribute from USB to core")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/bf83354747b8abaa486fe03af4be2826363469cb.1632740376.git.mchehab+huawei@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

PCI: Add sysfs "removable" attribute

A PCI device is "external_facing" if it's a Root Port with the ACPI
"ExternalFacingPort" property or if it has the DT "external-facing"
property. We consider everything downstream from such a device to
be removable by user.

We're mainly concerned with consumer platforms with user accessible
Thunderbolt ports that are vulnerable to DMA attacks, and we expect those
ports to be identified by firmware as "ExternalFacingPort". Devices in
traditional hotplug slots can technically be removed, but the expectation
is that unless the port is marked with "ExternalFacingPort", such devices
are less accessible to user / may not be removed by end user, and thus not
exposed as "removable" to userspace.

This can be used to implement userspace policies tailored for
user removable devices. Eg usage:
https://chromium-review.googlesource.com/c/chromiumos/platform2/+/2591812
https://chromium-review.googlesource.com/c/chromiumos/platform2/+/2795038
(code uses such an attribute to remove external PCI devices or disable
features on them as needed by the policy desired)

Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Rajat Jain <rajatja@google.com>
Link: https://lore.kernel.org/r/20210524171812.18095-2-rajatja@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

driver core: Move the "removable" attribute from USB to core

Move the "removable" attribute from USB to core in order to allow it to be
supported by other subsystem / buses. Individual buses that want to support
this attribute can populate the removable property of the device while
enumerating it with the 3 possible values -
- "unknown"
- "fixed"
- "removable"
Leaving the field unchanged (i.e. "not supported") would mean that the
attribute would not show up in sysfs for that device. The UAPI (location,
symantics etc) for the attribute remains unchanged.

Move the "removable" attribute from USB to the device core so it can be
used by other subsystems / buses.

By default, devices do not have a "removable" attribute in sysfs.

If a subsystem or bus driver wants to support a "removable" attribute, it
should call device_set_removable() before calling device_register() or
device_add(), e.g.:

device_set_removable(dev, DEVICE_REMOVABLE);
device_register(dev);

The possible values and the resulting sysfs attribute contents are:

DEVICE_REMOVABLE_UNKNOWN -> "unknown"
DEVICE_REMOVABLE -> "removable"
DEVICE_FIXED -> "fixed"

Convert the USB "removable" attribute to use this new device core
functionality. There should be no user-visible change in the location or
semantics of attribute for USB devices.

Reviewed-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Rajat Jain <rajatja@google.com>
Link: https://lore.kernel.org/r/20210524171812.18095-1-rajatja@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>