linux-kernel - Re: [PATCH v5 7/9] tty: serial: atmel: Only divide Clock Divisor if the IP is USART

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Message-ID: <4071cac3-d106-6f5e-aefb-adefba7dc567@microchip.com>
Date:   Fri, 30 Sep 2022 07:22:50 +0000
From:   <Claudiu.Beznea@...rochip.com>
To:     <Sergiu.Moga@...rochip.com>, <lee@...nel.org>,
        <robh+dt@...nel.org>, <krzysztof.kozlowski+dt@...aro.org>,
        <Nicolas.Ferre@...rochip.com>, <alexandre.belloni@...tlin.com>,
        <radu_nicolae.pirea@....ro>, <richard.genoud@...il.com>,
        <gregkh@...uxfoundation.org>, <jirislaby@...nel.org>,
        <Kavyasree.Kotagiri@...rochip.com>
CC:     <devicetree@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <linux-spi@...r.kernel.org>,
        <linux-serial@...r.kernel.org>, <ilpo.jarvinen@...ux.intel.com>
Subject: Re: [PATCH v5 7/9] tty: serial: atmel: Only divide Clock Divisor if
 the IP is USART

On 22.09.2022 14:33, Sergiu Moga wrote:
> Make sure that the driver only divides the clock divisor if the
> IP handled at that point is USART, since UART IP's do not support
> implicit peripheral clock division. Instead, in the case of UART,
> go with the highest possible clock divisor.
> 
> Signed-off-by: Sergiu Moga <sergiu.moga@...rochip.com>
> Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>

Reviewed-by: Claudiu Beznea <claudiu.beznea@...rochip.com>


> ---
> 
> 
> v1 -> v2:
> - Nothing, this patch was not here before and is mainly meant as both cleanup
> and as a way to introduce a new field into struct atmel_uart_port that will be
> used by the last patch to diferentiate between USART and UART regarding the
> location of the Baudrate Clock Source bitmask.
> 
> 
> 
> v2 -> v3:
> - Use ATMEL_US_CD instead of 65535
> - Previously [PATCH 10]
> 
> 
> 
> v3 -> v4:
> - Use min_t instead of &
> - Previously [PATCH 12]
> 
> 
> v4 -> v5:
> - Added R-b tag
> 
> 
> 
>  drivers/tty/serial/atmel_serial.c | 17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/serial/atmel_serial.c b/drivers/tty/serial/atmel_serial.c
> index e3e14cb7668b..acbf6b82d687 100644
> --- a/drivers/tty/serial/atmel_serial.c
> +++ b/drivers/tty/serial/atmel_serial.c
> @@ -150,6 +150,7 @@ struct atmel_uart_port {
>  	u32			rts_low;
>  	bool			ms_irq_enabled;
>  	u32			rtor;	/* address of receiver timeout register if it exists */
> +	bool			is_usart;
>  	bool			has_frac_baudrate;
>  	bool			has_hw_timer;
>  	struct timer_list	uart_timer;
> @@ -1825,6 +1826,7 @@ static void atmel_get_ip_name(struct uart_port *port)
>  	 */
>  	atmel_port->has_frac_baudrate = false;
>  	atmel_port->has_hw_timer = false;
> +	atmel_port->is_usart = false;
>  
>  	if (name == new_uart) {
>  		dev_dbg(port->dev, "Uart with hw timer");
> @@ -1834,6 +1836,7 @@ static void atmel_get_ip_name(struct uart_port *port)
>  		dev_dbg(port->dev, "Usart\n");
>  		atmel_port->has_frac_baudrate = true;
>  		atmel_port->has_hw_timer = true;
> +		atmel_port->is_usart = true;
>  		atmel_port->rtor = ATMEL_US_RTOR;
>  		version = atmel_uart_readl(port, ATMEL_US_VERSION);
>  		switch (version) {
> @@ -1863,6 +1866,7 @@ static void atmel_get_ip_name(struct uart_port *port)
>  			dev_dbg(port->dev, "This version is usart\n");
>  			atmel_port->has_frac_baudrate = true;
>  			atmel_port->has_hw_timer = true;
> +			atmel_port->is_usart = true;
>  			atmel_port->rtor = ATMEL_US_RTOR;
>  			break;
>  		case 0x203:
> @@ -2286,10 +2290,21 @@ static void atmel_set_termios(struct uart_port *port,
>  		cd = uart_get_divisor(port, baud);
>  	}
>  
> -	if (cd > 65535) {	/* BRGR is 16-bit, so switch to slower clock */
> +	/*
> +	 * If the current value of the Clock Divisor surpasses the 16 bit
> +	 * ATMEL_US_CD mask and the IP is USART, switch to the Peripheral
> +	 * Clock implicitly divided by 8.
> +	 * If the IP is UART however, keep the highest possible value for
> +	 * the CD and avoid needless division of CD, since UART IP's do not
> +	 * support implicit division of the Peripheral Clock.
> +	 */
> +	if (atmel_port->is_usart && cd > ATMEL_US_CD) {
>  		cd /= 8;
>  		mode |= ATMEL_US_USCLKS_MCK_DIV8;
> +	} else {
> +		cd = min_t(unsigned int, cd, ATMEL_US_CD);
>  	}
> +
>  	quot = cd | fp << ATMEL_US_FP_OFFSET;
>  
>  	if (!(port->iso7816.flags & SER_ISO7816_ENABLED))