linux-kernel - Re: [PATCH v3 10/15] serial: stm32-usart: Add STM32 USART Driver

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Date:	Tue, 17 Mar 2015 19:56:47 +0200
From:	Andy Shevchenko <andy.shevchenko@...il.com>
To:	Maxime Coquelin <mcoquelin.stm32@...il.com>
Cc:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>,
	Andreas Färber <afaerber@...e.de>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
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Subject: Re: [PATCH v3 10/15] serial: stm32-usart: Add STM32 USART Driver

On Tue, Mar 17, 2015 at 7:32 PM, Maxime Coquelin
<mcoquelin.stm32@...il.com> wrote:
> 2015-03-13 15:19 GMT+01:00 Andy Shevchenko <andy.shevchenko@...il.com>:

>>> +static void stm32_set_termios(struct uart_port *port, struct ktermios *termios,
>>> +                           struct ktermios *old)
>>> +{
>>> +       unsigned int baud;
>>> +       u32 usardiv, mantissa, fraction;
>>> +       tcflag_t cflag;
>>> +       u32 cr1, cr2, cr3;
>>> +       unsigned long flags;
>>> +
>>> +       baud = uart_get_baud_rate(port, termios, old, 0, port->uartclk / 16);
>>> +       cflag = termios->c_cflag;
>>> +
>>> +       spin_lock_irqsave(&port->lock, flags);
>>> +
>>> +       /* Stop serial port and reset value */
>>> +       writel_relaxed(0, port->membase + USART_CR1);
>>> +
>>> +       cr1 = USART_CR1_TE | USART_CR1_RE | USART_CR1_UE | USART_CR1_RXNEIE;
>>> +
>>> +       if (cflag & CSTOPB)
>>> +               cr2 = USART_CR2_STOP_2B;
>>> +
>>> +       if (cflag & PARENB) {
>>> +               cr1 |= USART_CR1_PCE;
>>> +               if ((cflag & CSIZE) == CS8)
>>> +                       cr1 |= USART_CR1_M;
>>> +       }
>>> +
>>> +       if (cflag & PARODD)
>>> +               cr1 |= USART_CR1_PS;
>>> +
>>> +       if (cflag & CRTSCTS)
>>> +               cr3 = USART_CR3_RTSE | USART_CR3_CTSE;
>>> +
>>> +       usardiv = (port->uartclk * 25) / (baud * 4);
>>> +       mantissa = (usardiv / 100) << USART_BRR_DIV_M_SHIFT;
>>> +       fraction = DIV_ROUND_CLOSEST((usardiv % 100) * 16, 100);
>>> +       if (fraction & ~USART_BRR_DIV_F_MASK) {
>>> +               fraction = 0;
>>> +               mantissa += (1 << USART_BRR_DIV_M_SHIFT);
>>> +       }
>>
>> So, it's a fractional divider. right? Could it be then fractional
>> divider clock in this first place (see
>> drivers/clk/clk-fractional-divider.c)?
>>
>
> I'm not sure it makes sense to represent this baudrate divider within
> the UART IP as a clock.

You have it already. I mean it should be fractional divider clock.

stm32port->clk = devm_clk_get(&pdev->dev, NULL);
+
+       if (WARN_ON(IS_ERR(stm32port->clk)))
+               return -EINVAL;
+
+       /* ensure that clk rate is correct by enabling the clk */
+       clk_prepare_enable(stm32port->clk);
+       stm32port->port.uartclk = clk_get_rate(stm32port->clk);

> What would be the gain?

Remove custom implementation of the calculations. It will be just one
line to refresh the baud rate. I think we have a lot of duplicates
here and there in the kernel. It would be nice not to bring another
one.

-- 
With Best Regards,
Andy Shevchenko
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