Hi,

On Thu, Jan 14, 2016 at 11:40:15AM +0100, Marcus Weseloh wrote:

quoted

On Mon, Dec 28, 2015 at 06:31:32PM +0100, Marcus Weseloh wrote:

quoted

This patch fixes some problems in the mod0 clock calculation. It has
the potential to break stuff, as the issues explained below had the
effect that clk_set_rate would always return successfully, sometimes
setting a frequency that is higher than the requested value.

That's actually the expected behaviour of clk_set_rate.

clk_set_rate is supposed to adjust the given clock rate to something
that the clock drivers seems fit. It should only return an error in a
case where you can't change the rate at all (because you didn't pass a
valid struct clk pointer, because changing the rate would violate some
clock flags, etc.). Otherwise, clk_set_rate should succeed.

By returning an error code the clock is higher than the one passed,
you violate that expectation, especially since that is relative to the
clock you passed.

It makes sense in your case to never exceed the given rate, it might
not for a different clock in the tree, or even for a different
instance of the same clock. For example, you could very well have
another case in your system where you should not have rates set that
are below the one given because that would prevent the consumer
device to be usable.

This is why the adjustment is left to the clock driver, and is not
enforced by the framework itself, simply because the framework has no
idea how you want to round your clock rate on that particular clock in
your system.

I understand now, thanks a lot for the good explanation! So my
thinking is wrong for the general case of the clock framework itself,
and that actually makes a lot of sense.

But the clk_factors_determine_rate function in
drivers/clk/sunxi/clk-factors.c works on the assumption that the
returned rate must be less or equal to the requested rate. At least
that is what the code in that function tries to do. That the mod0
factor calculation doesn't check the m and div variables for overflow
undermines the intended behaviour, as it "lies" about the frequencies
that the hardware can support.

Yeah, that's totally something that needs fixing.

And for very low frequencies below 80kHz, clk_set_rate does
currently return -EINVAL. There are even cases when it results in a
division by zero error, for example if you request a rate of 94kHz
from a 24Mhz parent (24000000 / 94000 = 255,32, rounded up to 256 =
0 on the u8 variable).

Yeah, the division by 0 is bad... However, can you even generate 80kHz
frequencies using a module clock? The lowest I can see here is 180kHz
(24M / 8 / 16).

Now I'm unsure what to do here... If the clock driver should only
return an error in real error cases and not when the requested
frequency isn't reachable, then clk_factors_determine_rate needs to
be changed as well?

If it returns an error in such a case, yeah. Also, I think in such a
case, we can simply round to the minimal frequency we can reach
(without an error or a kernel panic, that would be ideal ;))

quoted

quoted

An example:
parent_rate = 24Mhz, freq = 1.4Mhz results in p=1, m=9, freq=1333333,3333
(which gets rounded down to 1333333).
Calling the function again with parent_rate = 24Mhz and freq = 1333333
results in p=1, m=10, freq=1200000.

Rounding up the returned frequency removes this problem.

Signed-off-by: Marcus Weseloh <redacted>
---
 drivers/clk/sunxi/clk-mod0.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/clk/sunxi/clk-mod0.c b/drivers/clk/sunxi/clk-mod0.c
index d167e1e..d03f099 100644
--- a/drivers/clk/sunxi/clk-mod0.c
+++ b/drivers/clk/sunxi/clk-mod0.c

@@ -31,7 +31,8 @@
 static void sun4i_a10_get_mod0_factors(u32 *freq, u32 parent_rate,
                                     u8 *n, u8 *k, u8 *m, u8 *p)
 {
-     u8 div, calcm, calcp;
+     unsigned int div, calcm;
+     u8 calcp;

      /* These clocks can only divide, so we will never be able to achieve
       * frequencies higher than the parent frequency */

@@ -50,8 +51,10 @@ static void sun4i_a10_get_mod0_factors(u32 *freq, u32 parent_rate,
              calcp = 3;

      calcm = DIV_ROUND_UP(div, 1 << calcp);
+     if (calcm > 16)
+             calcm = 16;

-     *freq = (parent_rate >> calcp) / calcm;
+     *freq = DIV_ROUND_UP(parent_rate >> calcp, calcm);

While the two above seems harmless, this one concerns me a bit. Did
you test the various mod0 clock users and made sure that they were
still working as they used to?

No, I didn't do a thorough test, only booted the board with some mod0
users (mmc, spi, ss) and watched them request their frequencies
successfully. But this is an edge case, only affecting certain "weird"
frequencies. And the only effect is that the chosen frequency is not
the optimal one. So maybe I should drop it because it looks too
disruptive for too little gain?

I'm guessing the other changes are fine. I'm a bit worried about this
one, but we still have quite some time before the 4.6 release. We can
always merge it, and deal with the fallout.

Thanks!
Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com
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