Fractional divider on the Atmel USART controller

[Ludovic Desroches]

On Mon, Feb 06, 2017 at 04:37:23PM +0100, Ludovic Desroches wrote:
> On Mon, Feb 06, 2017 at 03:58:52PM +0100, Richard Genoud wrote:
> > 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 :
> > 
> 
> Right, it seems there was an update of all sama5 datasheet to remove
> this constraint. I don't know if it concerns also old devices, I'll try
> to get more information.
> 

The restriction to normal mode is no longer needed for all products.
Datasheet of old SoCs have not been updated.

Limiting the fractional baudrate was too restrictive. It works with
other modes but there is a warning (section 44.10.24 in SAMA5D2
datasheet):
"Warning: When the value of field FP is greater than 0, the SCK
(oversampling clock) generates non-constant duty cycles.
The SCK high duration is increased by “selected clock” period from time
to time. The duty cycle depends on the value of
the CD field."

In some situations, it may cause problems especially with ISO7816 mode.

> > 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 ?
> > > 

I think you can propose this as a patch. Adding the datasheet warning as
comment and commit message could be helpful.

Regards

Ludovic

> > > 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) ?
> > 
> 
> SAMA5D4: 0x701
> 
> Regards
> 
> Ludovic