drivers: PL011: add ARM SBSA Generic UART support

This is the fourth revision of the SBSA UART support series, now based on the current tty-next branch.
Compared to v3 I addressed some comments Jakub had on v3 (thanks for the review!). Also since some basic ACPI support for ARM64 is now upstream, I include the necessary ACPI binding as a new patch 11 on top.
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The ARM Server Base System Architecture[1] document describes a generic UART which is a subset of the ARM PL011 UART.
It lacks DMA support, baud rate control and modem status line control, among other things.
The idea is to move the UART initialization and setup into the firmware (which does this job today already) and let the kernel just use the UART for sending and receiving characters.

This patchset integrates support for this UART subset into the existing PL011 driver - basically by refactoring some functions and providing a new uart_ops structure for it. It also has a separate probe function to be not dependent on AMBA/PrimeCell. It provides a device tree and an ACPI binding.
Beside the obvious effect of code sharing reusing most of the PL011 code has the advantage of not introducing another serial device prefix, so it can go with ttyAMA, which seems to be pretty common. As changing the baudrate and other communication parameters is not specified for the SBSA UART, any userland attempt will be denied by the kernel. The device-tree provided fixed baud rate will be reported, so stty for instance prints the right value.

This series is based on Greg's tty-next branch, a git repo can be found at [2] (branch sbsa-uart/v4).

Patch 1/11 contains a bug fix which applies to the PL011 part also, it should be considered regardless of the rest of the series. According to Russell this is still racy, but still better than the current solution.
Patch 2-7 refactor some PL011 functions by splitting them up into smaller pieces, so that most of the code can be reused later by the SBSA part.
Patch 8 and 9 introduce two new properties for the vendor structure, this is for SBSA functionality which cannot be controlled by separate uart_ops members only.
Patch 10 then finally drops in the SBSA specific code, by providing a new uart_ops, vendor struct and probe function for it. Also the new device tree binding is documented.
Patch 11 adds the necessary ACPI boilerplate.

For testing you should be able to take any hardware which has a PL011 and change the DT to use a "arm,sbsa-uart" compatible string and the baud rate with the "current-speed" property.
Of course testing with a real SBSA Generic UART is welcomed - as well as regression testing with any PL011 implementation.
I tested this on a FastModel, a Juno and a Midway machine, both in PL011 and in (DT-emulated) SBSA mode.

Changelog:
v3 .. v4:
- fixed rebase artifact
- moved memory allocation out of pl011_allocate_port() function (and consequently renamed that to pl011_find_free_port)
- added ACPI driver binding

v2 .. v3:
- rebased on top of tty-next and Dave's latest PL011 rework
- fixed module build
- removed redundant =NULL members in sbsa_uart_ops

v1 .. v2:
- rebased on top of 4.0-rc1 and Dave's newest PL011 fix [3]
- added mandatory current-speed property and report that to userland

Cheers,
Andre

[1] ARM-DEN-0029 Server Base System Architecture, available (click- thru...) from http://infocenter.arm.com
[2] http://www.linux-arm.org/git?p=linux-ap.git
git://linux-arm.org/linux-ap.git
[3] http://lists.infradead.org/pipermail/linux-arm-kernel/2015-March/327631.html

Andre Przywara (10):
drivers: PL011: avoid potential unregister_driver call
drivers: PL011: refactor pl011_startup()
drivers: PL011: refactor pl011_shutdown()
drivers: PL011: refactor pl011_set_termios()
drivers: PL011: refactor pl011_probe()
drivers: PL011: replace UART_MIS reading with _RIS & _IMSC
drivers: PL011: move cts_event workaround into separate function
drivers: PL011: allow avoiding UART enabling/disabling
drivers: PL011: allow to supply fixed option string
drivers: PL011: add support for the ARM SBSA generic UART

Graeme Gregory (1):
drivers: PL011: add ACPI probing for SBSA UART

 .../devicetree/bindings/serial/arm_sbsa_uart.txt   |  10 +
 drivers/tty/serial/amba-pl011.c                    | 547 +++++++++++++++------
 2 files changed, 409 insertions(+), 148 deletions(-)
 create mode 100644 Documentation/devicetree/bindings/serial/arm_sbsa_uart.txt