[U-Boot] [PATCHv1] mx53ppd: Change UART clock divider for high baudrate

[Rick Bronson]

Hi Fabio,

  First a disclaimer.  I no longer work for the client that had me
make this change.  I also no longer have any iMX53 hardware to do any
kind of tests.  But I'm happy to help out with this email.

> Hi Rick and Sebastian,
>
> On Thu, Feb 15, 2018 at 2:15 PM, Sebastian Reichel
> <sebastian.reichel at collabora.co.uk> wrote:
> > From: Rick Bronson <rick at efn.org>
> >
> > Allow higher UART baud rates for communications to ACHC at 13.5 MBaud
>
> What does ACHC mean?

  ACHC is an product acronym, probably should change that sentence to:

Allow higher UART baud rates for communications up to 13.5 MBaud

> > Done in u-boot since it changes a divider common to all UARTs, including
> > the console. If it were done in Linux we would get garbled characters
> > during the handoff from u-boot to the kernel.
>
> It would be nice to explain a bit how you achieve the higher baud rates.

  Background: PLL2 runs at 400 MHz, PLL3 runs at 216 MHz.  One of
these PLL's are fed to a UART divider, the output of which sets the
maximum UART baud rates that are based on these rates divided by 16.

  Before patch: PLL3 (400 MHz) for UART's, divide by 6, get 66.66 MHz
from it.  Max baud rate is 4.166 MBaud.

  After patch: PLL2 (216 MHz), divide by 1, get 216 MHz from it.  Max
baud rate is 13.5 MBaud.

  The last I saw this code was 2014 so it's a little fuzzy.  Not sure
why I didn't keep PLL3 and just change the divider from 6 to 3, that
would have allowed a max of 8.33 MBaud and at the time, I was just
looking for at least 8 MBaud.

>
> Looks like you don't use PLL2 as the UART clock anyomore.

  That's correct, see above.

>
> Please expland a bit further on the new UART clock sheme.
>
> >
> > Signed-off-by: Sebastian Reichel <sebastian.reichel at collabora.co.uk>
> > ---
> >  arch/arm/mach-imx/mx5/lowlevel_init.S | 4 ++++
> >  1 file changed, 4 insertions(+)
> >
> > diff --git a/arch/arm/mach-imx/mx5/lowlevel_init.S
b/arch/arm/mach-imx/mx5/lowlevel_init.S
> > index f5bc6728b7c2..ee4c3afd2fed 100644
> > --- a/arch/arm/mach-imx/mx5/lowlevel_init.S
> > +++ b/arch/arm/mach-imx/mx5/lowlevel_init.S
> > @@ -319,11 +319,13 @@ setup_pll_func:
> >         ldr r1, =0x00016154
> >         str r1, [r0, #CLKCTL_CBCMR]
> >
> > +#if !defined(CONFIG_TARGET_MX53PPD)
>
> It would be better if this option could be a generic one, something
> like CONFIG_MX53_SERIAL_HIGHBAUD, so that other boads could use it if
> needed.

  Yes it probably would.

  Cheers,

  Rick