arm: rockchip: rk3399: Program PLL clock for DDR at 50 MHz in documented range

arm: rockchip: rk3399: Program PLL clock for DDR at 50 MHz in documented range

The original code set up the DDR clock to 48 MHz, not 50MHz as
requested, and did it in a way that didn't satisfy the Application
Notes in RK3399 TRM [1]. 2.9.2.B says:

   PLL frequency range requirement
   [...]
   FOUTVCO: 800MHz to 3.2GHz

2.9.2.A :
   PLL output frequency configuration
   [...]
   FOUTVCO = FREF / REFDIV * FBDIV
   FOUTPOSTDIV = FOUTVCO / POSTDIV1 / POSTDIV2

FREF = 24 MHz

The original code gives FOUTVCO: 24MHz/1 * 12 = 288MHz < 800MHz
And the resulting FOUTPOSTDIV is 288MHz / 3 / 2 = 48MHz
but the requested frequency was 50MHz

Note:
2.7.2 Detail Register Description
PMUCRU_PPLL_CON0 says

   fbdiv
   Feedback Divide Value
   Valid divider settings are:
   [16, 3200] in integer mode

So .fbdiv = 12 wouldn't be right. But 2.9.2.C says:

   PLL setting consideration
   [...]
   The following settings are valid for FBDIV:
   DSMPD=1 (Integer Mode):
   12,13,14,16-4095 (practical value is limited to 3200, 2400, or 1600
   (FVCOMAX / FREFMIN))
   [...]

So .fbdiv = 12 would be right.

In any case FOUTVCO is still wrong. I thank YouMin Chen for
confirmation and explanation.

Despite documentation, I don't seem to be able to reproduce a
practical problem with the wrong FOUTVCO. When I initially found it I
thought some problems with detecting the RAM capacity in my Rock Pi 4B
could be related to it and my patch seemed to help. But since I'm no
longer able to reproduce the issue, it works with or without this
patch. And meanwhile a patch[2] by Lee Jones and YouMin Chen addresses
this issue. Btw, shouldn't that be commited?

So this patches solves no visible problem.  Yet, to prevent future
problems, I think it'd be best to stick to spec.

An alternative to this patch could be

    {.refdiv = 1, .fbdiv = 75, .postdiv1 = 6, .postdiv2 = 6};

This would theoretically consume more power and yield less jitter,
according to 2.9.2.C :

   PLL setting consideration
   [...]
   For lowest power operation, the minimum VCO and FREF frequencies
   should be used. For minimum jitter operation, the highest VCO and
   FREF frequencies should be used.
   [...]

But I haven't tried it because I don't think it matters much. 50MHz
for DDR is only shortly used by TPL at RAM init. Normal operation is
at 800MHz.  Maybe it's better to use less power until later when more
complex software can control batteries or charging or whatever ?

Cc: Simon Glass <sjg@chromium.org>
Cc: Philipp Tomsich <philipp.tomsich@vrull.eu>
Cc: Kever Yang <kever.yang@rock-chips.com>
Cc: Lukasz Majewski <lukma@denx.de>
Cc: Sean Anderson <seanga2@gmail.com>
Link: [1] https://opensource.rock-chips.com/images/e/ee/Rockchip_RK3399TRM_V1.4_Part1-20170408.pdf
Link: [2] https://patchwork.ozlabs.org/project/uboot/list/?series=305766

Signed-off-by: Xavier Drudis Ferran <xdrudis@tinet.cat>
Tested-by: Michal Suchánek <msuchanek@suse.de>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>