linux-kernel - Re: [PATCH] atmel_serial: Use the fractional divider when possible

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Date:   Mon, 13 Feb 2017 09:07:29 +0100
From:   Richard Genoud <richard.genoud@...il.com>
To:     Romain Izard <romain.izard.pro@...il.com>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Richard Genoud <richard.genoud@...il.com>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>
Cc:     Ludovic Desroches <ludovic.desroches@...rochip.com>
Subject: Re: [PATCH] atmel_serial: Use the fractional divider when possible

2017-02-10 16:49 GMT+01:00 Ludovic Desroches <ludovic.desroches@...rochip.com>:
> On Fri, Feb 10, 2017 at 04:24:46PM +0100, Romain Izard wrote:
>> The fractional baud rate generator is available when using the
>> asynchronous mode of Atmel USART controllers. It makes it possible to
>> use higher baudrates, in exchange for a less precise clock with a
>> variable duty cycle.
>>
>> The existing code restricts its use to the normal mode of the USART
>> controller, following the recommendation from the datasheet for the
>> first chip embedding this type of controller. This recommendation has
>> been removed from the documentation for the newer chips. After
>> verification, all revisions of this controller should be able to use the
>> fractional baud rate generator with the different asynchronous modes.
>>
>> Removing the condition on ATMEL_US_USMODE makes it possible to get
>> correct baudrates at high speed in more cases.
>>
>> This was tested with a board using an Atmel SAMA5D2 chip and a TI
>> WL1831 WiFi/Bluetooth combo chip at 3 Mbauds, with hardware flow control
>> enabled.
>>
>> Signed-off-by: Romain Izard <romain.izard.pro@...il.com>
>
> Acked-by: Ludovic Desroches <ludovic.desroches@...rochip.com>

Signed-off-by: Richard Genoud <richard.genoud@...il.com>

Thanks !
> Thanks
>
>> ---
>>  drivers/tty/serial/atmel_serial.c | 7 ++++---
>>  1 file changed, 4 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/tty/serial/atmel_serial.c b/drivers/tty/serial/atmel_serial.c
>> index fabbe76203bb..6684456dca9e 100644
>> --- a/drivers/tty/serial/atmel_serial.c
>> +++ b/drivers/tty/serial/atmel_serial.c
>> @@ -1758,7 +1758,9 @@ static void atmel_get_ip_name(struct uart_port *port)
>>
>>       /*
>>        * Only USART devices from at91sam9260 SOC implement fractional
>> -      * baudrate.
>> +      * baudrate. It is available for all asynchronous modes, with the
>> +      * following restriction: the sampling clock's duty cycle is not
>> +      * constant.
>>        */
>>       atmel_port->has_frac_baudrate = false;
>>       atmel_port->has_hw_timer = false;
>> @@ -2202,8 +2204,7 @@ static void atmel_set_termios(struct uart_port *port, struct ktermios *termios,
>>        * then
>>        * 8 CD + FP = selected clock / (2 * baudrate)
>>        */
>> -     if (atmel_port->has_frac_baudrate &&
>> -         (mode & ATMEL_US_USMODE) == ATMEL_US_USMODE_NORMAL) {
>> +     if (atmel_port->has_frac_baudrate) {
>>               div = DIV_ROUND_CLOSEST(port->uartclk, baud * 2);
>>               cd = div >> 3;
>>               fp = div & ATMEL_US_FP_MASK;
>> --
>> 2.9.3
>>