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Userspace applications need to know the maximum supported message size. The cdc-wdm driver translates between a character device stream and a message based protocol. Each message is transported as a usb control message with no further encapsulation or syncronization. Each read or write on the character device should translate to exactly one usb control message to ensure that message boundaries are kept intact. That means that the userspace application must know the maximum message size supported by the device and driver, making this size a vital part of the cdc-wdm character device API. CDC WDM and CDC MBIM functions export the maximum supported message size through CDC functional descriptors. The cdc-wdm and cdc_mbim drivers will parse these descriptors and use the value chosen by the device. The only current way for a userspace application to retrive the value is by duplicating the descriptor parsing. This is an unnecessary complex task, and application writers are likely to postpone it, using a fixed value and adding a "todo" item. QMI functions have no way to tell the host what message size they support. The qmi_wwan driver use a fixed value based on protocol recommendations and observed device behaviour. Userspace applications must know and hard code the same value. This scheme will break if we ever encounter a QMI device needing a device specific message size quirk. We are currently unable to support such a device because using a non default size would break the implicit userspace API. The message size is currently a hidden attribute of the cdc-wdm userspace API. Retrieving it is unnecessarily complex, increasing the possibility of drivers and applications using different limits. The resulting errors are hard to debug, and can only be replicated on identical hardware. Exporting the maximum message size from the driver simplifies the task for the userspace application, and creates a unified information source independent of device and function class. It also serves to document that the message size is part of the cdc-wdm userspace API. This proposed API extension has been presented for the authors of userspace applications and libraries using the current API: libmbim, libqmi, uqmi, oFono and ModemManager. The replies were: Aleksander Morgado: "We do really need max message size for MBIM; and as you say, it may be good to have the max message size info also for QMI, so the new ioctl seems a good addition. So +1 from my side, for what it's worth." Dan Williams: "Yeah, +1 here. I'd prefer the sysfs file, but the fact that that doesn't work for fd passing pretty much kills it." No negative replies are so far received. Cc: Aleksander Morgado <aleksander@lanedo.com> Cc: Dan Williams <dcbw@redhat.com> Signed-off-by: Bjørn Mork <bjorn@mork.no> Acked-by: Oliver Neukum <oliver@neukum.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> |
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