[1/3] USB: core: Unite old scheme and new scheme descriptor reads

In preparation for reworking the usb_get_device_descriptor() routine,
it is desirable to unite the two different code paths responsible for
initially determining endpoint 0's maximum packet size in a newly
discovered USB device.  Making this determination presents a
chicken-and-egg sort of problem, in that the only way to learn the
maxpacket value is to get it from the device descriptor retrieved from
the device, but communicating with the device to retrieve a descriptor
requires us to know beforehand the ep0 maxpacket size.

In practice this problem is solved in two different ways, referred to
in hub.c as the "old scheme" and the "new scheme".  The old scheme
(which is the approach recommended by the USB-2 spec) involves asking
the device to send just the first eight bytes of its device
descriptor.  Such a transfer uses packets containing no more than
eight bytes each, and every USB device must have an ep0 maxpacket size
>= 8, so this should succeed.  Since the bMaxPacketSize0 field of the
device descriptor lies within the first eight bytes, this is all we
need.

The new scheme is an imitation of the technique used in an early
Windows USB implementation, giving it the happy advantage of working
with a wide variety of devices (some of them at the time would not
work with the old scheme, although that's probably less true now).  It
involves making an initial guess of the ep0 maxpacket size, asking the
device to send up to 64 bytes worth of its device descriptor (which is
only 18 bytes long), and then resetting the device to clear any error
condition that might have resulted from the guess being wrong.  The
initial guess is determined by the connection speed; it should be
correct in all cases other than full speed, for which the allowed
values are 8, 16, 32, and 64 (in this case the initial guess is 64).

The reason for this patch is that the old- and new-scheme parts of
hub_port_init() use different code paths, one involving
usb_get_device_descriptor() and one not, for their initial reads of
the device descriptor.  Since these reads have essentially the same
purpose and are made under essentially the same circumstances, this is
illogical.  It makes more sense to have both of them use a common
subroutine.

This subroutine does basically what the new scheme's code did, because
that approach is more general than the one used by the old scheme.  It
only needs to know how many bytes to transfer and whether or not it is
being called for the first iteration of a retry loop (in case of
certain time-out errors).  There are two main differences from the
former code:

	We initialize the bDescriptorType field of the transfer buffer
	to 0 before performing the transfer, to avoid possibly
	accessing an uninitialized value afterward.

	We read the device descriptor into a temporary buffer rather
	than storing it directly into udev->descriptor, which the old
	scheme implementation used to do.

Since the whole point of this first read of the device descriptor is
to determine the bMaxPacketSize0 value, that is what the new routine
returns (or an error code).  The value is stored in a local variable
rather than in udev->descriptor.  As a side effect, this necessitates
moving a section of code that checks the bcdUSB field for SuperSpeed
devices until after the full device descriptor has been retrieved.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Cc: Oliver Neukum <oneukum@suse.com>

---

 drivers/usb/core/hub.c |  173 ++++++++++++++++++++++++++-----------------------
 1 file changed, 94 insertions(+), 79 deletions(-)

Message ID	495cb5d4-f956-4f4a-a875-1e67e9489510@rowland.harvard.edu (mailing list archive)
State	Accepted
Commit	85d07c55621676d47d873d2749b88f783cd4d5a1
Headers	show Return-Path: <linux-usb-owner@vger.kernel.org> Date: Fri, 4 Aug 2023 15:10:59 -0400 From: Alan Stern <stern@rowland.harvard.edu> To: Greg KH <gregkh@linuxfoundation.org> Cc: Khazhy Kumykov <khazhy@google.com>, Oliver Neukum <oneukum@suse.com>, USB mailing list <linux-usb@vger.kernel.org> Subject: [PATCH 1/3] USB: core: Unite old scheme and new scheme descriptor reads Message-ID: <495cb5d4-f956-4f4a-a875-1e67e9489510@rowland.harvard.edu> References: <6eadec91-990a-4fbd-8883-8366c4a4d8e4@rowland.harvard.edu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <6eadec91-990a-4fbd-8883-8366c4a4d8e4@rowland.harvard.edu> Precedence: bulk
Series	USB: core: Don't overwrite device descriptor during reinitialization \| expand [0/3] USB: core: Don't overwrite device descriptor during reinitialization [1/3] USB: core: Unite old scheme and new scheme descriptor reads [2/3] USB: core: Change usb_get_device_descriptor() API [3/3] USB: core: Fix race by not overwriting udev->descriptor in hub_port_init()

[1/3] USB: core: Unite old scheme and new scheme descriptor reads

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