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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> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.