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The driver will forward errors to userspace after turning most of them into -EIO. But all status codes are not equal. The -EPIPE (stall) in particular can be seen more as a result of normal USB signaling than an actual error. The state is automatically cleared by the USB core without intervention from either driver or userspace. And most devices and firmwares will never trigger a stall as a result of GetEncapsulatedResponse. This is in fact a requirement for CDC WDM devices. Quoting from section 7.1 of the CDC WMC spec revision 1.1: The function shall not return STALL in response to GetEncapsulatedResponse. But this driver is also handling GetEncapsulatedResponse on behalf of the qmi_wwan and cdc_mbim drivers. Unfortunately the relevant specs are not as clear wrt stall. So some QMI and MBIM devices *will* occasionally stall, causing the GetEncapsulatedResponse to return an -EPIPE status. Translating this into -EIO for userspace has proven to be harmful. Treating it as an empty read is safer, making the driver behave as if the device was conforming to the CDC WDM spec. There have been numerous reports of issues related to -EPIPE errors from some newer CDC MBIM devices in particular, like for example the Fibocom L831-EAU. Testing on this device has shown that the issues go away if we simply ignore the -EPIPE status. Similar handling of -EPIPE is already known from e.g. usb_get_string() The -EPIPE log message is still kept to let us track devices with this unexpected behaviour, hoping that it attracts attention from firmware developers. Cc: <stable@vger.kernel.org> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100938 Reported-and-tested-by: Christian Ehrig <christian.ehrig@mediamarktsaturn-bt.com> Reported-and-tested-by: Patrick Chilton <chpatrick@gmail.com> Reported-and-tested-by: Andreas Böhler <news@aboehler.at> Signed-off-by: Bjørn Mork <bjorn@mork.no> Acked-by: Oliver Neukum <oneukum@suse.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. 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.