[U-Boot] [PATCHv2] fsl_ddr: Don't use full 64-bit divides on32-bit PowerPC

[York Sun]

Kyle,

On Fri, 2011-07-29 at 12:20 -0500, Moffett, Kyle D wrote:
> On Jul 28, 2011, at 17:41, York Sun wrote:
> > On Mon, 2011-03-14 at 16:35 -0500, Moffett, Kyle D wrote:
> >> On Mar 14, 2011, at 16:22, York Sun wrote:
> >>> On Wed, 2011-02-23 at 11:35 -0500, Kyle Moffett wrote:
> >>>> +	 * Now divide by 5^12 and track the 32-bit remainder, then divide
> >>>> +	 * by 2*(2^12) using shifts (and updating the remainder).
> >>>> +	 */
> >>>> +	clks_rem = do_div(clks, UL_5pow12);
> >>>> +	clks_rem <<= 13;
> >>> 
> >>> Shouldn't this be clks_rem >>= 13 ?
> >>>> 
> >>>> +	clks_rem |= clks & (UL_2pow13-1);
> >>>> +	clks >>= 13;
> >>>> +
> >>>> +	/* If we had a remainder, then round up */
> >>>> +	if (clks_rem)
> >>>> 		clks++;
> >>>> -	}
> > 
> > I found a problem with the round up. Please try
> > 
> > picos_to_mclk(mclk_to_picos(3))
> > 
> > You will notice the result is round up. I am sure it is unexpected.
> 
> Hmm... what does fsl_ddr_get_mem_data_rate() return on your system?
> I cannot reproduce this here with my memory freq of 800MHz.
> 
> The function has obviously always done rounding, even before I made
> any changes.  Have you retested what the math does with the patch
> reverted to see if it makes any difference?
> 
> Hmm, looking at it, the obvious problem case would be mclk_to_picos()
> using a rounded result in a subsequent multiply.  Can you retest by
> replacing mclk_to_picos() with the following code; this should keep
> the error down by doing the rounding-to-10ps *after* the multiply.
> 
> unsigned int mclk_to_picos(unsigned int mclk)
> {
>         unsigned int data_rate = get_ddr_freq(0);
>         unsigned int result;
> 
>         /* Round to nearest 10ps, being careful about 64-bit multiply/divide */
>         unsigned long long mclk_ps = ULL_2e12 * mclk;
> 
>         /* Add 5*data_rate, for rounding */
>         mclk_ps += 5*(unsigned long long)data_rate;
>  
>         /* Now perform the big divide, the result fits in 32-bits */
>         do_div(mclk_ps, data_rate);
>         result = mclk_ps;
> 
>         /* We still need to round to 10ps */
>         return 10 * (result/10);
> }
> 
> Obviously you could also get rid of the rounding alltogether, but I
> don't know why it was put in the code in the first place...

I think the problem comes from the round up. But your calculation of
remainder is not right. You explained to me with example of 2*10^^12-1.
It seems to be right until I tried another number 264*10^^7. I think the
following calculation of remainder is

	clks_rem = do_div(clks, UL_5POW12);
	clks >>= 13;
	clks_rem |= (clks & (UL_2POW13-1)) * UL_5POW12;

Then we can compare the remainder with the "error" introduced by
mclk_to_picos, which is 10ps.

	if (clks_rem > 10 * data_rate)
		clk++;

What do you think?

York