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
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