[PATCH v2 4/8] tty: serial: samsung: Init USI to keep clocks running

[Krzysztof Kozlowski]

On 06/08/2021 17:21, Sam Protsenko wrote:
> 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.
> 
> Signed-off-by: Sam Protsenko <semen.protsenko at linaro.org>
> ---
> Changes in v2:
>   - Non-intrusive modification of USI registers
>   - Improved comments
>   - Rearranged USI register definitions to conform with existing style
> 
>  drivers/tty/serial/samsung_tty.c | 32 +++++++++++++++++++++++++++++++-
>  include/linux/serial_s3c.h       |  9 +++++++++
>  2 files changed, 40 insertions(+), 1 deletion(-)
> 


Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski at canonical.com>



Best regards,
Krzysztof