[U-Boot] [PATCH] nand_base: Add timeout for NAND reset command

[Peter Tyser <ptyser@xes-inc.com>]

On Wed, 2009-02-04 at 13:54 -0600, Scott Wood wrote:
> Peter Tyser wrote:
> > +/*
> > + * CONFIG_SYS_NAND_RESET_CNT is used as a timeout mechanism when resetting
> > + * a flash.  NAND flash is initialized prior to interrupts so standard timers
> > + * can't be used.  CONFIG_SYS_NAND_RESET_CNT should be set to a value
> > + * which is greater than (max NAND reset time / NAND status read time).
> > + * A conservative default of 200000 (500 us / 25 ns) is used as a default.
> > + */
> > +#ifndef CONFIG_SYS_NAND_RESET_CNT
> > +#define CONFIG_SYS_NAND_RESET_CNT 200000
> > +#endif
> 
> Where does 25 ns come from?  Should the timeout be in terms of real time 
> rather than iterations (we use get_ticks() for this purpose in 
> fsl_elbc_nand.c)?

The 25ns was calculated based on the addition of trp and trhoh from the
Micron MT29F8G08 datasheet.  Based on the timing diagram for a "Read
Status" cycle I thought this would be the minimum cycle time needed to
read the chip's status.  Other chips (ST, Samsung) I glanced at had > 25
ns read status times as well.

I had tried using get_timer() (I believe nand_wait() would have been
perfect to use), but that didn't work due to interrupts being disabled
when NAND is probed.  I didn't consider using get_ticks()...  That seems
much better.  Is get_ticks() available for all platforms when NAND is
initialized?

Assuming get_ticks() is available for all platforms, would you prefer I:
1. re-do the patch using get_ticks()
2. update nand_wait() to use get_ticks instead of get_timer() and use it

Thanks,
Peter