serial: mvebu-uart: implement UART clock driver for configuring UART base clock

Implement a new device driver for controlling UART clocks on Marvell
Armada 3700 SoC. This device driver is loaded for devices which match
the compatible string "marvell,armada-3700-uart-clock".

There are more pitfalls related to UART clocks:
- both UARTs use same parent clock source (which can be xtal or one of
  the TBG clocks),
- if a TBG clock is used as the parent clock, there are two additional
  divisors that can both be configured to divide the rate by 1, 2, ... 6,
  but these divisors are again shared between the two UART controllers
  on the SOC,
- the configuration of the parent clock source and divisors is done in
  the address space of the first UART controller, UART1. Clocks can be
  gated separately for UART1 and UART2, but this setting also lives in
  the address space of UART1,
- Marvell's Functional Specification for Armada 3720 document has the
  clock gating bits swapped, so the one described to gate UART1 clock
  actually gates UART2 and vice versa,
- each UART has it's own "special divisor", and this uses the parent
  clock described above. These divisors are configure in each UART's
  address space separately.

Thus the driver for UART2 controller needs to have access to UART1
address space, since UART1 address space contains some bits exclusive
for UART2 and also some bits which are shared between UART1 and UART2.

Also, during boot, when early console is active on one of the UARTs,
and we want to switch parent clock from xtal (default) to TBG (to be
more flexible with baudrates), the driver changing UART clocks also
needs to be able to change the "special divisor", so that the baudrate
of earlycon is not changed when swtiching to normal console. Thus the
clock driver also needs to be able to access UART2 register space,
for UART2's "special divisor".

For these reasons, this new UART clock driver does not use
ioremap_resource(), but only ioremap() to prevent resource conflicts
between UART clock driver and UART driver.

We need to share only two 32-bit registers between the UART driver and
the UART clock driver:
- UART Clock Control
- UART 2 Baud Rate Divisor
Access to these two registers are protected by one spinlock to prevent
any conflicts. Access is required only during probing, when changing
baudrate or during suspend/resume.

Hardware can be configured to use one of following clocks as UART parent
clock: TBG-A-P, TBG-B-P, TBG-A-S, TBG-B-S, xtal. Not every clock is
usable for higher buadrates. Any subset can be specified in the
device-tree and the driver will choose the best one which also still
supports the mandatory baudrate of 9600 Bd. For smooth boot log output
it is needed to specify clock used by early console, otherwise garbage
would be printed on UART during probe of UART clock driver and
transitioning from early console to normal console.

We are implementing this to be able to configure TBG clock as UART
parent clock, which is required to be able to achieve higher baudrates
than 230400 Bd. We achieve this by referencing this new UART clock
device node in UART's device node. UART clock device driver
automatically chooses the best clock source for UART driver.

Until now, UART's device-tree node needed to reference one of the static
clocks (xtal or one of the TBGs) as parent clock in the `clocks`
phandle - the parent clock which was configured before booting the
kernel. If bootloader changed UART's parent clock, it needed to change
the `clocks` phandle in DTB correspondingly before booting.

From now on both the old mechanism (xtal or TBG referenced as parent
clock in `clocks` phandle) and the new one (UART clock referenced in the
`clocks` phandle) are supported, to provide full backward compatibility
with existing DTS files, full backward compatibility with existing boot
loaders, and to provide new features (runtime clock configuration to
allow higher baudrates than 230400 Bd). New features are available only
with new DTS files.

There was also a discussion about how the UART node and the
clock-controller node could be wrapped together in a new binding [1, 2].
As explained there, this is not possible if we want to keep backwards
compatibility with existing bootloaders, and thus we are doing this by
putting the UART clock-controller node inside the UART1 node.

[1] https://lore.kernel.org/linux-serial/20220120000651.in7s6nazif5qjkme@pali/
[2] https://lore.kernel.org/linux-serial/20220125204006.A6D09C340E0@smtp.kernel.org/

Reviewed-by: Marek Behún <kabel@kernel.org>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marek Behún <kabel@kernel.org>
Link: https://lore.kernel.org/r/20220219152818.4319-4-kabel@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 1 insertions, 0 deletions

diff --git a/drivers/tty/serial/Kconfig b/drivers/tty/serial/Kconfig
index 407a98ec0791..66916397a19c 100644
--- a/drivers/tty/serial/Kconfig
+++ b/drivers/tty/serial/Kconfig
@@ -1446,6 +1446,7 @@ config SERIAL_STM32_CONSOLE
 config SERIAL_MVEBU_UART
 	bool "Marvell EBU serial port support"
 	depends on ARCH_MVEBU || COMPILE_TEST
+	depends on COMMON_CLK
 	select SERIAL_CORE
 	help
 	  This driver is for Marvell EBU SoC's UART. If you have a machine