Linux kernel source tree
85d07c5562
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> Link: https://lore.kernel.org/r/495cb5d4-f956-4f4a-a875-1e67e9489510@rowland.harvard.edu Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.