Am 03.05.2013 22:47, schrieb Troy Kisky:
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.
Yes. I was looking for the _highest_ clock possible, though ;) And this isn't correctly done by the recent code. This is why I was looking into it ...
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;
Using my test code gives the correct values using this algorithm. So yes, sounds good.
Just a small note: Wouldn't it be better to put the printf and the last line with the pre_div calculation into the if(post_div > 4) part? I.e.
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;
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;
} else post_div = 0;
?
In case we agree on this, I'm thinking about doing 2 patches to make clear what we are doing:
1. Re-doing my initial patch with
post_div = pre_div / 16; pre_div = 16;
This would be the "fix the issues in the existing (non-optimal) algorithm but keep that" patch.
2. Replace the existing algorithm with your above version. This would be the "improve the algorithm" patch.
What do you think?
Best regards
Dirk