Re: [PATCH 04/12] tty: serial: samsung: Init USI to keep clocks running

[Sam Protsenko <semen.protsenko@linaro.org>]

On Fri, 30 Jul 2021 at 19:32, Krzysztof Kozlowski
<krzysztof.kozlowski@canonical.com> wrote:
>
> On 30/07/2021 16:49, 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@linaro.org>
> > ---
> >  drivers/tty/serial/samsung_tty.c | 33 +++++++++++++++++++++++++++++++-
> >  include/linux/serial_s3c.h       |  9 +++++++++
> >  2 files changed, 41 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/tty/serial/samsung_tty.c b/drivers/tty/serial/samsung_tty.c
> > index 9fbc61151c2e..0f3cbd0b37e3 100644
> > --- a/drivers/tty/serial/samsung_tty.c
> > +++ b/drivers/tty/serial/samsung_tty.c
> > @@ -65,6 +65,7 @@ enum s3c24xx_port_type {
> >  struct s3c24xx_uart_info {
> >       char                    *name;
> >       enum s3c24xx_port_type  type;
> > +     unsigned int            has_usi;
> >       unsigned int            port_type;
> >       unsigned int            fifosize;
> >       unsigned long           rx_fifomask;
> > @@ -1352,6 +1353,29 @@ static int apple_s5l_serial_startup(struct uart_port *port)
> >       return ret;
> >  }
> >
> > +static void exynos_usi_init(struct uart_port *port)
> > +{
> > +     struct s3c24xx_uart_port *ourport = to_ourport(port);
> > +     struct s3c24xx_uart_info *info = ourport->info;
> > +
> > +     if (!info->has_usi)
> > +             return;
> > +
> > +     /*
> > +      * USI_RESET is an active High signal. Reset value of USI_RESET is 0x1
> > +      * to drive stable value to PAD. Due to this feature, the USI_RESET must
> > +      * be cleared (set as 0x0) before starting a transaction.
>
> "before starting a transaction" suggests it is related with transaction
> or something before starting it. Don't you need it simply after reset or
> resume?
>

Not sure what you are suggesting. USI_RESET is set to "1" at start up
(means USI block hangs in reset state), so we have to make it "0"
(that's what this code does); only then UART becomes functional and
UART transactions can be performed. And exynos_usi_init() is called
exactly where you hinted: at init and on resume.

Anyway, the whole comment is confusing, I'll simplify and rework it in
v2. Please let me know if I'm missing the point though.

> > +      */
> > +     wr_regl(port, USI_CON, USI_RESET);
>
> You are clearing entire register, not only USI_RESET bitfield. Is it
> really what you want?
>

Yeah, USI_CON[31:1] bits are reserved, and the reset value of this
register is 0x00000001. But anyway, I'm going to rework that code like
this, for clarity and consistence:

8<--------------------------------------------------------------------->8
    /* Clear the software reset of USI block (it's set at startup) */
    val = rd_regl(port, USI_CON);
    val &= ~(USI_RESET_MASK)
    wr_regl(port, USI_CON, val);
    udelay(1);

    /* Continuously provide the clock to USI IP w/o gating (for Rx mode) */
    val = rd_regl(port, USI_OPTION);
    val &= ~USI_HWACG_MASK;
    val |= USI_HWACG_CLKREQ_ON;
    wr_regl(port, USI_OPTION, val);
8<--------------------------------------------------------------------->8

> > +     udelay(1);
> > +
> > +     /*
> > +      * Set the HWACG option bit in case of UART Rx mode.
> > +      * CLKREQ_ON = 1, CLKSTOP_ON = 0 (set USI_OPTION[2:1] = 0x1).
> > +      */
> > +     wr_regl(port, USI_OPTION, USI_HWACG_CLKREQ_ON);
> > +}
> > +
> >  /* power power management control */
> >
> >  static void s3c24xx_serial_pm(struct uart_port *port, unsigned int level,
> > @@ -1379,6 +1403,7 @@ static void s3c24xx_serial_pm(struct uart_port *port, unsigned int level,
> >               if (!IS_ERR(ourport->baudclk))
> >                       clk_prepare_enable(ourport->baudclk);
> >
> > +             exynos_usi_init(port);
> >               break;
> >       default:
> >               dev_err(port->dev, "s3c24xx_serial: unknown pm %d\n", level);
> > @@ -2102,6 +2127,8 @@ static int s3c24xx_serial_init_port(struct s3c24xx_uart_port *ourport,
> >       if (ret)
> >               pr_warn("uart: failed to enable baudclk\n");
> >
> > +     exynos_usi_init(port);
> > +
> >       /* Keep all interrupts masked and cleared */
> >       switch (ourport->info->type) {
> >       case TYPE_S3C6400:
> > @@ -2750,10 +2777,11 @@ static struct s3c24xx_serial_drv_data s5pv210_serial_drv_data = {
> >  #endif
> >
> >  #if defined(CONFIG_ARCH_EXYNOS)
> > -#define EXYNOS_COMMON_SERIAL_DRV_DATA                                \
> > +#define EXYNOS_COMMON_SERIAL_DRV_DATA_USI(_has_usi)          \
> >       .info = &(struct s3c24xx_uart_info) {                   \
> >               .name           = "Samsung Exynos UART",        \
> >               .type           = TYPE_S3C6400,                 \
> > +             .has_usi        = _has_usi,                     \
> >               .port_type      = PORT_S3C6400,                 \
> >               .has_divslot    = 1,                            \
> >               .rx_fifomask    = S5PV210_UFSTAT_RXMASK,        \
> > @@ -2773,6 +2801,9 @@ static struct s3c24xx_serial_drv_data s5pv210_serial_drv_data = {
> >               .has_fracval    = 1,                            \
> >       }                                                       \
> >
> > +#define EXYNOS_COMMON_SERIAL_DRV_DATA                                \
> > +     EXYNOS_COMMON_SERIAL_DRV_DATA_USI(0)
> > +
> >  static struct s3c24xx_serial_drv_data exynos4210_serial_drv_data = {
> >       EXYNOS_COMMON_SERIAL_DRV_DATA,
> >       .fifosize = { 256, 64, 16, 16 },
> > diff --git a/include/linux/serial_s3c.h b/include/linux/serial_s3c.h
> > index f6c3323fc4c5..013c2646863e 100644
> > --- a/include/linux/serial_s3c.h
> > +++ b/include/linux/serial_s3c.h
> > @@ -28,6 +28,15 @@
> >  #define S3C2410_UFSTAT         (0x18)
> >  #define S3C2410_UMSTAT         (0x1C)
> >
> > +/* USI Control Register offset */
> > +#define USI_CON                      (0xC4)
> > +/* USI Option Register offset */
> > +#define USI_OPTION           (0xC8)
> > +/* USI_CON[0] = 0b0: clear USI global software reset (Active High) */
> > +#define USI_RESET            (0<<0)
>
> Just 0x0. I understand you wanted to hint it is a bit field, but the
> shift of 0 actually creates more questions.
>

After some consideration I decided to adhere to existing style and do
something like this (in v2):

8<--------------------------------------------------------------------->8
#define USI_CON          (0xC4)
#define USI_OPTION      (0xC8)

#define USI_CON_RESET_CLEAR        (0<<0)
#define USI_CON_RESET_SET        (1<<0)
#define USI_CON_RESET_MASK        (1<<0)

#define USI_OPTION_HWACG_CLKREQ_ON    (1<<1)
#define USI_OPTION_HWACG_CLKSTOP_ON    (1<<2)
#define USI_OPTION_HWACG_MASK        (3<<1)
8<--------------------------------------------------------------------->8

The whole reason for those comments was missing public TRM. But in the
end I decided it just looks ugly. Also, this way I can do RMW
operation (discussed above) in more logical way.

Please let me know if code snippets above look good to you.

>
> Best regards,
> Krzysztof