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UART block is a part of USI (Universal Serial Interface) IP-core in
Samsung SoCs since Exynos9810 (e.g. in Exynos850). USI allows one to
enable one of three types of serial interface: UART, SPI or I2C. That's
possible because USI shares almost all internal circuits within each
protocol. USI also provides some additional registers so it's possible
to configure it.
One USI register called USI_OPTION has reset value of 0x0. Because of
this the clock gating behavior is controlled by hardware (HWACG =
Hardware Auto Clock Gating), which simply means the serial won't work
after reset as is. In order to make it work, USI_OPTION[2:1] bits must
be set to 0b01, so that HWACG is controlled manually (by software).
Bits meaning:
- CLKREQ_ON = 1: clock is continuously provided to IP
- CLKSTOP_ON = 0: drive IP_CLKREQ to High (needs to be set along with
CLKREQ_ON = 1)
USI is not present on older chips, like s3c2410, s3c2412, s3c2440,
s3c6400, s5pv210, exynos5433, exynos4210. So the new boolean field
'.has_usi' was added to struct s3c24xx_uart_info. USI registers will be
only actually accessed when '.has_usi' field is set to "1".
This feature is needed for further serial enablement on Exynos850, but
some other new Exynos chips (like Exynos9810) may benefit from this
feature as well.
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Link: https://lore.kernel.org/r/20210811114827.27322-5-semen.protsenko@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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.