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  • Analysis of Replicant 9 Linux 5.2 modem branch

Analysis of Replicant 9 Linux 5.2 modem branch¶

These were cleaned up and ported from forkbomb's code:

b4c2df98ed6f misc: xmm6262: Add Samsung IPC USB modem firmware download module
77b55273bd6e net: usb: add Samsung IPC-over-HSIC driver
f9ae2d1697fa net: add Samsung IPC interface driver
7806adad4507 HACK: usb: host: ehci-exynos: add ehci_power sysfs node
82c317b0da5e HACK: add modem power on/off driver
28e5b460f920 HACK: usb: ehci_exynos: enable OHCI_SUSP_LEGACY
e7c122b770c1 ARM: dts: EXYNOS: add 3G modem nodes to midas boards
c3201527f7ea ARM: dts: split Exynos 4412 N710x boards up
5b55f03cda8c ARM: dts: EXYNOS: add Samsung IPC modem support
eca381876d5e ARM: dts: EXYNOS: enable HSIC0 on midas boards
7ec9129e3221 replicant_*_defconfig: extend cmdline to get IMSI and other information
54c196380507 replicant_*_defconfig: Add modem support
[...]
0ecfebd2b524 Linux 5.2

If we look at the drivers only (not the dts or config changes, or hacks in pre-existing code, we are left with):

b4c2df98ed6f misc: xmm6262: Add Samsung IPC USB modem firmware download module
77b55273bd6e net: usb: add Samsung IPC-over-HSIC driver
f9ae2d1697fa net: add Samsung IPC interface driver
82c317b0da5e HACK: add modem power on/off driver

We'd also need to find a way to fix these:

7806adad4507 HACK: usb: host: ehci-exynos: add ehci_power sysfs node
28e5b460f920 HACK: usb: ehci_exynos: enable OHCI_SUSP_LEGACY

So we'd probably need to have something like that instead:
1. A driver for the SIPC (Samsung IPC) protocol (f9ae2d1697fa net: add Samsung IPC interface driver)
2. A driver for the SIPC (Samsung IPC) transport on top of HSIC and the protocol (77b55273bd6e net: usb: add Samsung IPC-over-HSIC driver).
3. A modem driver with:

• The GPIO handling like drivers/hsi/clients/nokia-modem.c (some GPIOs probably cannot be handled in userspace like gpio_pda_active (see XMMBoot for more details) (82c317b0da5e HACK: add modem power on/off driver)
• The firmware loading code (b4c2df98ed6f misc: xmm6262: Add Samsung IPC USB modem firmware download module)
• Probably some of the hacks here. For instance GPIO input IRQ -> hsic bus reset

So we have: Userspace <-> SIPC protocol <-> SIPC transport <-> HSIC

However I'm not sure about certain things:

• The SIPC protocol is huge and has different userspace interfaces. Maybe it could be split?
• The SIPC transport driver uses an USB id table (the modem ID once booted). => Should we keep the IDs in it or move them in the modem driver instead?

Also look at the motorolla cpcap driver for the droid4. The architecture is different though: While it uses USB, it's tied to Qualcomm modem drivers (CDC) which probably doesn't have a specific protocol beside the multiplexing of the UART lines and network interfaces