[U-Boot] [PATCH] spi: mxc_spi: Fix pre and post divider calculation

[Troy Kisky <troy.kisky@boundarydevices.com>]

On 5/2/2013 10:58 PM, Dirk Behme wrote:
> Do you want to say you propose
>
> post_div = pre_div / 16;
> pre_div = 16;
>
> ?
yes, that's what I said
>
> If so:
>
> First, I agree that we have to use the same dividers in both lines.
>
> But, second, this would mean that you use /16 as max pre_div. For the 
> i.MX6 case where clk_src is 60MHz this would result in a pre-divided 
> clock of 3.75Mhz (instead of 4MHz with /15).

That does sound better for i.MX6, what about other processors using this 
file?

>
> So using /15 or /16 is just a decision of which end clocks most 
> probably are needed.
>
> If you want to be able to configure 4MHz, 2MHz, 1MHz, 500kHz etc then 
> /15 is the better choice.
>
> If you want to be able to configure 3.75Mhz, 1.875MHz, 937.5kHz, 
> 468.75kHz etc then /16 is the better choice.
>
> I vote for /15 as done by my patch.

Thanks for explaining. The downside of using /15 is that you can't get 
the slowest clock possible.
How about restructuring the code to improve both. Calculate post_div first.

pre_div = DIV_ROUND_UP(clk_src, max_hz);
/* fls(1) = 1, fls(0x80000000) = 32, fls(16) = 5 */
post_div = fls(pre_div - 1);
if (post_div > 4)
     post_div -= 4;
else
     post_div = 0;

if (post_div >= 16) {
            printf("Error: no divider for the freq: %d\n",
                                         max_hz);
            return -1;
}
pre_div = (pre_div + (1 << post_div) - 1) >> post_div;
_________________________
Checking values for largest divisor possible (16 << 15) = (1<<19) gives
post_div = 15;
pre_div = 16;

Checking 1st illegal divisor (0x80001) gives
post_div = 16;

Checking divisor 0xe0 gives
post_div = 4;
pre_div = 0xe;

Checking divisor 0xe1 gives
post_div = 4;
pre_div = 0xf;

Checking divisor 0x100 gives
post_div = 4;
pre_div = 0x10;

Checking divisor 0x101 gives
post_div = 5;
pre_div = 9;
_________________________________________
Code is simpler and more accurate

Troy