Hi,
On Mon, Jun 6, 2022 at 11:19 AM Vijaya Krishna Nivarthi
<quic_vnivarth@xxxxxxxxxxx> wrote:
Hi,OK. ...I guess my question would be: does it matter for some reason?
On 6/4/2022 12:10 AM, Doug Anderson wrote:
Hi,I am afraid this still doesn't guarantee that ser_clk is a multiple of
On Fri, Jun 3, 2022 at 10:43 AM Vijaya Krishna Nivarthi
<quic_vnivarth@xxxxxxxxxxx> wrote:
Ah, or I guess what you're saying is that the table historicallyHow about just this (untested):
contained "rounded" rates but that clk_round_rate() isn't returning
nice round rates. OK, but if we truly want to support an inexact
match, you'd want to pick the rate that reduces the error, not just
pick the first one. In other words, something like this (untested):
freq = clk_round_rate(clk, mult);
diff = abs(((long)mult - freq) / div);
if (diff < best_diff) {
best_diff = diff;
ser_clk = freq;
best_div = div;
}
I am not sure if its required that freq is a multiple of best_div now
that we don't have a multiple of desired_clk anyway.
freq = clk_round_rate(clk, mult);
candidate_div = max(1, DIV_ROUND_CLOSEST(freq, desired_clk));
candidate_freq = freq / candidate_div;
diff = abs((long)desired_clk - candidate_freq);
if (diff < best_diff) {
best_diff = diff;
ser_clk = freq;
best_div = candidate_div;
}
best_div
"ser_clk" is just a local variable in this function. Who cares if it's
not a multiple of best_div? This is why we're keeping track of
"best_div" in the first place, so that later in the function instead
of:
*clk_div = ser_clk / desired_clk;
if (!(*clk_div))
*clk_div = 1;
You just do:
*clk_div = best_div;
Actually power saving was the anticipation behind returning first frequency in original patch, when we cant find exact frequency.
I tested it with a function simulates clk_round_rate.That doesn't look like a terrible result. I guess nominally 602 is a
static unsigned long clk_round_rate_test(struct clk *clk, unsigned long
in_freq)
{
unsigned long root_freq[6] = {105, 204, 303, 402, 501, 602};
int i;
for (i = 0; i < 6; i++) {
if (root_freq[i] >= in_freq)
return root_freq[i];
}
return root_freq[6];
}
{
unsigned long ser_clk;
unsigned long desired_clk;
unsigned long freq;
int div_round_closest;
unsigned long div;
unsigned long mult;
unsigned long candidate_div, candidate_freq;
unsigned long diff, best_diff, best_div;
unsigned long one;
desired_clk = 100;
one = 1;
best_diff = ULONG_MAX;
pr_err("\ndesired_clk-%d\n", desired_clk);
for (div = 1; div <= 10; div++) {
mult = div * desired_clk;
freq = clk_round_rate_test(clk, mult);
div_round_closest = DIV_ROUND_CLOSEST(freq, desired_clk);
candidate_div = max(one, (unsigned long)div_round_closest);
candidate_freq = freq / candidate_div;
diff = abs((long)desired_clk - candidate_freq);
pr_err("div-%d, mult-%d, freq-%d, div_round_closest-%d,
candidate_div-%d, candidate_freq-%d, diff-%d\n",
div, mult, freq, div_round_closest, candidate_div,
candidate_freq, diff);
if (diff < best_diff) {
pr_err("This is best so far\n");
best_diff = diff;
ser_clk = freq;
best_div = candidate_div;
}
}
pr_err("\nbest_diff-%d, ser_clk-%d, best_div-%d\n",
best_diff, ser_clk, best_div);
}
And here is the output
[ 17.835167] desired_clk-100
[ 17.839567] div-1, mult-100, freq-105, div_round_closest-1,
candidate_div-1, candidate_freq-105, diff-5
[ 17.849220] This is best so far
[ 17.852458] div-2, mult-200, freq-204, div_round_closest-2,
candidate_div-2, candidate_freq-102, diff-2
[ 17.862104] This is best so far
[ 17.865345] div-3, mult-300, freq-303, div_round_closest-3,
candidate_div-3, candidate_freq-101, diff-1
[ 17.874995] This is best so far
[ 17.878237] div-4, mult-400, freq-402, div_round_closest-4,
candidate_div-4, candidate_freq-100, diff-0
[ 17.887882] This is best so far
[ 17.891118] div-5, mult-500, freq-501, div_round_closest-5,
candidate_div-5, candidate_freq-100, diff-0
[ 17.900770] div-6, mult-600, freq-602, div_round_closest-6,
candidate_div-6, candidate_freq-100, diff-0
[ 17.910415] div-7, mult-700, freq-602, div_round_closest-6,
candidate_div-6, candidate_freq-100, diff-0
[ 17.920057] div-8, mult-800, freq-602, div_round_closest-6,
candidate_div-6, candidate_freq-100, diff-0
[ 17.929703] div-9, mult-900, freq-602, div_round_closest-6,
candidate_div-6, candidate_freq-100, diff-0
[ 17.939353] div-10, mult-1000, freq-602, div_round_closest-6,
candidate_div-6, candidate_freq-100, diff-0
[ 17.949181]
[ 17.949181] best_diff-0, ser_clk-402, best_div-4
better approximation, but if we're accepting that we're not going to
have an exact rate anyway then maybe being off by that tiny amount
doesn't matter and we'd do better with the slow clock (maybe saves
power?)
Please note that we go past cases when we have an divider that canAh, good point. Luckily that's a 1-line fix, right?
exactly divide the frequency(105/1, 204/2, 303/3) and end up with one
that doesn't.
-Doug