The Dell Wireless 5821e module is a Qualcomm SDX20 based LTE Cat16 device. This modem can work in either MBIM mode or QMI mode, and provides different USB layouts for each of the modes. In Linux kernel based and Windows based systems, the MBIM mode is the default one, because it provides easy integration with the OS (e.g. no additional drivers or connection managers required in Windows) and also provides all the features that QMI provides through QMI over MBIM operations.
The firmware update process of this DW5821e module is integrated in your GNU/Linux distribution, since ModemManager 1.10.0 and fwupd 1.2.6. There is no official firmware released in the LVFS (yet) but the setup is completely ready to be used, just waiting for Dell to publish an initial official firmware release.
The firmware update integration between ModemManager and fwupd involves different steps, which I’ll try to describe here so that it’s clear how to add support for more devices in the future.
1) ModemManager reports expected update methods, firmware version and device IDs
The Firmware interface in the modem object exposed in DBus contains, since MM 1.10, a new UpdateSettings property that provides a bitmask specifying which is the expected firmware update method (or methods) required for a given module, plus a dictionary of key-value entries specifying settings applicable to each of the update methods.
In the case of the DW5821e, two update methods are reported in the bitmask: “fastboot” and “qmi-pdc“, because both are required to have a complete firmware upgrade procedure. “fastboot” would be used to perform the system upgrade by using an OTA update file, and “qmi-pdc” would be used to install the per-carrier configuration files after the system upgrade has been done.
The list of settings provided in the dictionary contain the two mandatory fields required for all devices that support at least one firmware update method: “device-ids” and “version”. These two fields are designed so that fwupd can fully rely on them during its operation:
- The “device-ids” field will include a list of strings providing the device IDs associated to the device, sorted from the most specific to the least specific. These device IDs are the ones that fwupd will use to build the GUIDs required to match a given device to a given firmware package. The DW5821e will expose four different device IDs:
- “USB\VID_413C“: specifying this is a Dell-branded device.
- “USB\VID_413C&PID_81D7“: specifying this is a DW5821e module.
- “USB\VID_413C&PID_81D7&REV_0318“: specifying this is hardware revision 0x318 of the DW5821e module.
- “USB\VID_413C&PID_81D7&REV_0318&CARRIER_VODAFONE“: specifying this is hardware revision 0x318 of the DW5821e module running with a Vodafone-specific carrier configuration.
- The “version” field will include the firmware version string of the module, using the same format as used in the firmware package files used by fwupd. This requirement is obviously very important, because if the format used is different, the simple version string comparison used by fwupd (literally ASCII string comparison) would not work correctly. It is also worth noting that if the carrier configuration is also versioned, the version string should contain not only the version of the system, but also the version of the carrier configuration. The DW5821e will expose a firmware version including both, e.g. “T77W968.F22.214.171.124.1.VF.001” (system version being F126.96.36.199.1 and carrier config version being “VF.001”)
- In addition to the mandatory fields, the dictionary exposed by the DW5821e will also contain a “fastboot-at” field specifying which AT command can be used to switch the module into fastboot download mode.
2) fwupd matches GUIDs and checks available firmware versions
Once fwupd detects a modem in ModemManager that is able to expose the correct UpdateSettings property in the Firmware interface, it will add the device as a known device that may be updated in its own records. The device exposed by fwupd will contain the GUIDs built from the “device-ids” list of strings exposed by ModemManager. E.g. for the “USB\VID_413C&PID_81D7&REV_0318&CARRIER_VODAFONE” device ID, fwupd will use GUID “b595e24b-bebb-531b-abeb-620fa2b44045”.
fwupd will then be able to look for firmware packages (CAB files) available in the LVFS that are associated to any of the GUIDs exposed for the DW5821e.
The CAB files packaged for the LVFS will contain one single firmware OTA file plus one carrier MCFG file for each supported carrier in the give firmware version. The CAB files will also contain one “metainfo.xml” file for each of the supported carriers in the released package, so that per-carrier firmware upgrade paths are available: only firmware updates for the currently used carrier should be considered. E.g. we don’t want users running with the Vodafone carrier config to get notified of upgrades to newer firmware versions that aren’t certified for the Vodafone carrier.
Each of the CAB files with multiple “metainfo.xml” files will therefore be associated to multiple GUID/version pairs. E.g. the same CAB file will be valid for the following GUIDs (using Device ID instead of GUID for a clearer explanation, but really the match is per GUID not per Device ID):
- Device ID “USB\VID_413C&PID_81D7&REV_0318&CARRIER_VODAFONE” providing version “T77W968.F188.8.131.52.2.VF.002”
- Device ID “USB\VID_413C&PID_81D7&REV_0318&CARRIER_TELEFONICA” providing version “T77W968.F184.108.40.206.2.TF.003”
- Device ID “USB\VID_413C&PID_81D7&REV_0318&CARRIER_VERIZON” providing version “T77W968.F220.127.116.11.2.VZ.004”
- … and so on.
Following our example, fwupd will detect our device exposing device ID “USB\VID_413C&PID_81D7&REV_0318&CARRIER_VODAFONE” and version “T77W968.F18.104.22.168.1.VF.001” in ModemManager and will be able to find a CAB file for the same device ID providing a newer version “T77W968.F22.214.171.124.2.VF.002” in the LVFS. The firmware update is possible!
3) fwupd requests device inhibition from ModemManager
In order to perform the firmware upgrade, fwupd requires full control of the modem. Therefore, when the firmware upgrade process starts, fwupd will use the new InhibitDevice(TRUE) method in the Manager DBus interface of ModemManager to request that a specific modem with a specific uid should be inhibited. Once the device is inhibited in ModemManager, it will be disabled and removed from the list of modems in DBus, and no longer used until the inhibition is removed.
The inhibition may be removed by calling InhibitDevice(FALSE) explicitly once the firmware upgrade is finished, and will also be automatically removed if the program that requested the inhibition disappears from the bus.
4) fwupd downloads CAB file from LVFS and performs firmware update
Once the modem is inhibited in ModemManager, fwupd can right away start the firmware update process. In the case of the DW5821e, the firmware update requires two different methods and two different upgrade cycles.
The first step would be to reboot the module into fastboot download mode using the AT command specified by ModemManager in the “at-fastboot” entry of the “UpdateSettings” property dictionary. After running the AT command, the module will reset itself and reboot with a completely different USB layout (and different vid:pid) that fwupd can detect as being the same device as before but in a different working mode. Once the device is in fastboot mode, fwupd will download and install the OTA file using the fastboot protocol, as defined in the “flashfile.xml” file provided in the CAB file:
<parts interface="AP"> <part operation="flash" partition="ota" filename="T77W968.F126.96.36.199.2.AP.123_ota.bin" MD5="f1adb38b5b0f489c327d71bfb9fdcd12"/> </parts>
Once the OTA file is completely downloaded and installed, fwupd will trigger a reset of the module also using the fastboot protocol, and the device will boot from scratch on the newly installed firmware version. During this initial boot, the module will report itself running in a “default” configuration not associated to any carrier, because the OTA file update process involves fully removing all installed carrier-specific MCFG files.
The second upgrade cycle performed by fwupd once the modem is detected again involves downloading all carrier-specific MCFG files one by one into the module using the QMI PDC protocol. Once all are downloaded, fwupd will activate the specific carrier configuration that was previously active before the download was started. E.g. if the module was running with the Vodafone-specific carrier configuration before the upgrade, fwupd will select the Vodafone-specific carrier configuration after the upgrade. The module would be reseted one last time using the QMI DMS protocol as a last step of the upgrade procedure.
5) fwupd removes device inhibition from ModemManager
The upgrade logic will finish by removing the device inhibition from ModemManager using InhibitDevice(FALSE) explicitly. At that point, ModemManager would re-detect and re-probe the modem from scratch, which should already be running in the newly installed firmware and with the newly selected carrier configuration.