Re: [PATCH] mtd-nand: davinci: correct 4-bit error correction

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 13 Jul 2010 15:02:59 +0530 "Sudhakar Rajashekhara" <sudhakar.raj@ti.com> wrote:

> Hi,
> 
> On Mon, Jul 12, 2010 at 11:58:18, Sudhakar Rajashekhara wrote:
> > On Sat, Jul 10, 2010 at 04:09:32, Andrew Morton wrote:
> > > On Fri,  9 Jul 2010 10:59:49 +0530
> > > Sudhakar Rajashekhara <sudhakar.raj@ti.com> wrote:
> > > 
> > > > +
> > > > +	/*
> > > > +	 * ECC_STATE field reads 0x3 (Error correction complete) immediately
> > > > +	 * after setting the 4BITECC_ADD_CALC_START bit. So if you immediately
> > > > +	 * begin trying to poll for the state, you may fall right out of your
> > > > +	 * loop without any of the correction calculations having taken place.
> > > > +	 * The recommendation from the hardware team is to wait till ECC_STATE
> > > > +	 * reads less than 4, which means ECC HW has entered correction state.
> > > > +	 */
> > > > +	do {
> > > > +		ecc_state = (davinci_nand_readl(info,
> > > > +				NANDFSR_OFFSET) >> 8) & 0x0f;
> > > > +		cpu_relax();
> > > > +	} while ((ecc_state < 4) && time_before(jiffies, timeo));
> > > 
> > > An up-to-100-milliseond busy wait is pretty bad.  For how long do you
> > > expect this to spin in practice?
> > 
> > On the hardware, I have never seen this taking 100 msec to come out of
> > the loop. I'll check with the hardware folks on the maximum time to wait
> > for, before the ECC engine is ready.
> 
> I checked this with the hardware team but no one is sure about the exact
> time one should wait before the ECC engine becomes ready. But everyone is
> of the opinion that 100 loop cycles should be enough. To be on the safer
> side, I'll be changing the timeout to 10 milliseconds in the next version
> of this patch.

Going from 100ms down to 10ms sounds a bit risky.  It'd be better to
retain the 100ms and to make the kernel spend most of that time
sleeping, rather than busywaiting, IMO.