Comment 0 for bug 1864147

[SRU Justification]

[Impact]
Support Intel Atom (Baytrail-I) HS-UART serdev slaves over tty

On systems using Intel Atom (Baytrail-I) SoC's, slave devices connected on
HSUART1/2 ports are described by the ACPI BIOS as virtual hardware using
HID's INT3511/INT3512.

As a consequence, HW manufacturers have complete freedom to install any
devices on-board as long as they can be accessed over serial tty
interface. Once such device is Dell Edge 3002 IoT Gateway which sports
ZigBee & GPS devices on the HS-UART ports 1 & 2 respectively.

Post 4.15, with CONFIG_SERIAL_DEV_BUS=y, serdev port controller driver
handles the enumeration for the slaves connected on these ports. Also,
/dev/ttySx device nodes for these ports are no longer exposed to the
userspace.

[Fixes]
This patchset allows the skipped virtual UART's with ACPI HID's mentioned
above to be enumerated as ttyHSx port.

[Regression Potential]
Low. This driver binds only to Intel Atom E38XX (Baytrail-I) virtual serial
ports on the Dell Edge 300x GW. These updates are being carried in the
linux-oem (4.15.y) kernel for more than 2 years without any regressions.

[Note]
This driver has been submitted upstream and is under review. This issue has
been identified as a regression in 4.15 and multiple solutions are being
explored. Meanwhile, to support customer and maintain release schedule,
this driver needs to be in Ubuntu 4.15 based kernels.
More info:
https://marc.info/?t=152455871600007&r=1&w=2

linux-oem buglink: http://bugs.launchpad.net/bugs/ 1690362