[U-Boot] [PATCH 04/14] sunxi: dram: Code cleanup and comments for the CKE delay handling

[Ian Campbell]

On Fri, 2014-07-25 at 04:41 +0300, Siarhei Siamashka wrote:
> On Mon, 21 Jul 2014 19:51:50 +0100
> Ian Campbell <ijc at hellion.org.uk> wrote:
> 
> > On Fri, 2014-07-18 at 19:22 +0300, Siarhei Siamashka wrote:
> > > Before driving the CKE pin (Clock Enable) high, the DDR3 spec requires
> > > to wait for additional 500 us after the RESET pin is de-asserted.
> > > 
> > > The DRAM controller takes care of this delay by itself, using a
> > > configurable counter in the SDR_IDCR register. This works in the same
> > > way on sun4i/sun5i/sun7i hardware (even the default register value
> > > 0x00c80064 is identical). Except that the counter is ticking a bit
> > > slower on sun7i (3 DRAM clock cycles instead of 2), resulting in
> > > longer actual delays for the same settings.
> > > 
> > > This patch keeps the old code and only removes the CONFIG_SUN7I ifdef.
> > > But maybe we should drop all of this and just add 'udelay(500)' after
> > > the DDR3 reset without bothering to play with these undocumented
> > > registers.
> > 
> > I'm happy to go with whichever you think is better.
> 
> If the total DRAM initialization time in u-boot is not really critical
> (all the delays are only fractions of millisecond), then I would
> probably go with the "cargo cult" approach and actually apply the
> delays in both places ('udelay(500)' after the DDR3 reset and keep
> the maximum delay in the SDR_IDCR register too).

Makes sense to me.

If someone later decides they really care about boot time to this degree
then they can implement the SDR_IDCR thing, hopefully with the aid of a
logic analyser, as you say.

Ian.