On Fri, Jul 01, 2016 at 02:50:57AM +0200, Ond?ej Jirman wrote:

quoted

quoted

 /**
+ * sun8i_h3_apply_pll1_factors() - applies n, k, m, p factors to the
+ * register using an algorithm that tries to reserve the PLL lock
+ */
+
+static void sun8i_h3_apply_pll1_factors(struct clk_factors *factors, struct factors_request *req)
+{
+	const struct clk_factors_config *config = factors->config;
+	u32 reg;
+
+	/* Fetch the register value */
+	reg = readl(factors->reg);
+
+	if (FACTOR_GET(config->pshift, config->pwidth, reg) < req->p) {
+		reg = FACTOR_SET(config->pshift, config->pwidth, reg, req->p);
+
+		writel(reg, factors->reg);
+		__delay(2000);
+	}

So there was some doubts about the fact that P was being used, or at
least that it was useful.

p is necessary to reduce frequencies below 288 MHz according to the
datasheet.

Yes, but you could reach those frequencies without P, too, and it's
not part of any OPP provided by Allwinner.

quoted

quoted

+	if (FACTOR_GET(config->mshift, config->mwidth, reg) < req->m) {
+		reg = FACTOR_SET(config->mshift, config->mwidth, reg, req->m);
+
+		writel(reg, factors->reg);
+		__delay(2000);
+	}
+
+	reg = FACTOR_SET(config->nshift, config->nwidth, reg, req->n);
+	reg = FACTOR_SET(config->kshift, config->kwidth, reg, req->k);
+
+	writel(reg, factors->reg);
+	__delay(20);
+
+	while (!(readl(factors->reg) & (1 << config->lock)));

So, they are applying the dividers first, and then applying the
multipliers, and then wait for the PLL to stabilize.

Not exactly, first we are increasing dividers if the new dividers are
higher that that what's already set. This ensures that because
application of dividers is immediate by the design of the PLL, the
application of multipliers isn't. So the VCO would still run at the same
frequency for a while gradually rising to a new value for example,
while the dividers would be reduced immediately. Leading to crash.

PLL
--------------------------
PRE DIV(f0) -> VCO(f1) -> POST DIV(f2)
   P             K,N           M

Example: (we set all factors at once, reducing dividers and multipliers
at the same time at 0ms - this should lead to no change in the output
frequency, but...)

-1ms: f0 = 24MHz, f1 = 2GHz,   f2 = 1GHz
 0ms: f0 = 24MHz, f1 = 2GHz,   f2 = 2GHz       - boom
 1ms: f0 = 24MHz, f1 = 1.5GHz, f2 = 1.5GHz
 2ms: f0 = 24MHz, f1 = 1GHz,   f2 = 1GHz

The current code crashes exactly at boom, you don't get any more
instructions to execute.

See.

So this patch first increases dividers (only if necessary), changes
multipliers and waits for change to happen (takes around 2000 cycles),
and then decreases dividers (only if necessary).

So we get:

-1ms: f0 = 24MHz, f1 = 2GHz,   f2 = 1GHz
 0ms: f0 = 24MHz, f1 = 2GHz,   f2 = 1GHz   - no boom, multiplier
                                             reduced
 1ms: f0 = 24MHz, f1 = 1.5GHz, f2 = 0.75GHz
1.9ms: f0 = 24MHz, f1 = 1GHz,   f2 = 0.5GHz - we got PLL sync
 2ms: f0 = 24MHz, f1 = 1GHz,   f2 = 1GHz   - and here we reduce divider
at last

Awesome explanation, thanks!

So I guess it really all boils down to the fact that the CPU is
clocked way outside of it's operating frequency while the PLL
stabilizes, right?

If so, then yes, trying to switch to the 24MHz oscillator before
applying the factors, and then switching back when the PLL is stable
would be a nice solution.

I just checked, and all the SoCs we've had so far have that
possibility, so if it works, for now, I'd like to stick to that.

quoted

quoted

+
+	if (FACTOR_GET(config->mshift, config->mwidth, reg) > req->m) {
+		reg = FACTOR_SET(config->mshift, config->mwidth, reg, req->m);
+
+		writel(reg, factors->reg);
+		__delay(2000);
+	}
+
+	if (FACTOR_GET(config->pshift, config->pwidth, reg) > req->p) {
+		reg = FACTOR_SET(config->pshift, config->pwidth, reg, req->p);
+
+		writel(reg, factors->reg);
+		__delay(2000);
+	}

However, this is kind of weird, why would you need to re-apply the
dividers? Nothing really changes. Have you tried without that part?

See above, we either increase before PLL change, or reduce dividers
after the change. Nothing is re-applied.

quoted

Since this is really specific, I guess you could simply make the
clk_ops for the nkmp clocks public, and just re-implement set_rate
using that logic.

I would argue that this may be necessary for other PLL clocks too, if
you can get out of bounds output frequency, by changing the dividers too
early or too late. So perhaps this code should be generalized for other
PLL clocks too, instead.

We can scale down the problem a bit. The only PLL we modify are the
CPU, audio and video ones.

The CPU should definitely be addressed, but what would be the outcome
of an out of bounds audio pll for example? Is it just taking more time
to stabilize, or would it hurt the system?

In the former case, then we can just wait. In the latter, we of course
need to come up with a solution.

Maxime


-- 
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 819 bytes
Desc: not available
URL: <http://lists.infradead.org/pipermail/linux-arm-kernel/attachments/20160715/d7b66f8b/attachment-0001.sig>