linux-kernel - Re: [PATCH v3 12/14] tty: serial: atmel: Only divide Clock Divisor if the IP is USART

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Date:   Wed, 14 Sep 2022 14:01:38 +0300 (EEST)
From:   Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To:     Sergiu Moga <sergiu.moga@...rochip.com>
cc:     lee@...nel.org, robh+dt@...nel.org,
        krzysztof.kozlowski+dt@...aro.org, nicolas.ferre@...rochip.com,
        alexandre.belloni@...tlin.com, claudiu.beznea@...rochip.com,
        radu_nicolae.pirea@....ro, richard.genoud@...il.com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        broonie@...nel.org, mturquette@...libre.com, sboyd@...nel.org,
        Jiri Slaby <jirislaby@...nel.org>, admin@...iphile.com,
        kavyasree.kotagiri@...rochip.com, tudor.ambarus@...rochip.com,
        devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        LKML <linux-kernel@...r.kernel.org>, linux-spi@...r.kernel.org,
        linux-serial <linux-serial@...r.kernel.org>,
        linux-clk@...r.kernel.org
Subject: Re: [PATCH v3 12/14] tty: serial: atmel: Only divide Clock Divisor
 if the IP is USART

On Tue, 13 Sep 2022, 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>
> ---
> 
> 
> 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]


> @@ -2283,10 +2287,21 @@ 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 &= ATMEL_US_CD;

Now that I read the above comment with more thought, would:
		cd = min(cd, ATMEL_US_CD);
be more appropriate here if "the highest possible value" is sought for?


-- 
 i.