Fractional divider on the Atmel USART controller

[Richard Genoud]

Hi Ludovic,

On 06/02/2017 14:42, Ludovic Desroches wrote:
> Hello Romain,
> 
> On Mon, Feb 06, 2017 at 12:56:42PM +0100, Romain Izard wrote:
>> Hello,
>>
>> On Atmel SAMA5D2, when trying to configure a serial port for 3 Mbauds
>> operation, I do not always get the requested baud rate. If the hardware
>> flow control is disabled by software, the line works correctly. But if I
>> set the crtscts option, the line does not work, and after checking the
>> line I can observe that the signal is sent at 2.6 Mbauds.
>>
>> This is due to the code used to manage fractional baud rate divisor: the
>> existing code prevents the fractional bits from being used if the line
>> is not configured in normal mode. This case occurs when the hardware
>> flow control or the RS485 mode is set.
>>
>> If I apply the following patch to drivers/tty/serial/atmel_serial.c,
>> I get the required baudrate.
>>
>> 8<----------------------------------------------------------------
>>
>> @@ -2204,14 +2204,13
>>       * baudrate = selected clock / (8 * (2 - OVER) * (CD + FP / 8))
>>       * Currently, OVER is always set to 0 so we get
>>       * baudrate = selected clock / (16 * (CD + FP / 8))
>>       * 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;
>>      } else {
>>          cd = uart_get_divisor(port, baud);
>>      }
>>
>> 8<----------------------------------------------------------------
>>
>> Unfortunately, I know that this will work on SAMA5D2, but this driver is
>> used for many other Atmel chips. I do not know if the existing code is
>> meant to respect a known limitation on other devices that use the same
>> controller, or if it is just a bug.
> 
> It depends on the device used. In the SAMA5D4 datasheet, it is
> mentionned: "This feature is only available when using USART normal mode".

It seems you have an old datasheet version, because in table 60.2 (page 1784) of SAMA5D4 datasheet (rev D), there's :

Section 43.6.1.2 “Fractional Baud Rate in Asynchronous Mode”: added warning “When the value of field FP
is greater than 0...”; removed sentence “This feature is only available when using USART normal mode.”

So I guess this should also work on SAMA5D4 ? 


> In the SAMA5D2 datasheet, this constraint is no longer mentionned you
> confirm that is works, that is great.
> 
>>
>> Ludovic, Nicolas,  what is your opinion on that matter? Should I just
>> propose this as a patch, or is it necessary to add a limitation for
>> supported devices only ?
> 
> Yes, only SAMA5D2 supports fractionnal baudrate in other modes than the
> normal one. A new compatible string may be added, I am not sure you have
> enough information in atmel_get_ip_name...

I checked the version numbers on some atmel platforms I have:

AT91SAM9G35 / SAMA5D3: 0x502
AT91SAM9G20: 0x10330
SAMA5D2: 0x814

Ludovic, could you please check the version number of the SAMA5D4 USART (if you have one lying around) ?