Changed: sunxi-ng clock code - NKMP clock implementation is wrong

[Maxime Ripard]

Hi Ondrej,

On Thu, Jul 28, 2016 at 01:27:05PM +0200, Ondřej Jirman wrote:
> Hi Maxime,
> 
> I don't have your sunxi-ng clock patches in my mailbox, so I'm replying
> to this.

You can find it in the clock maintainers tree:
https://git.kernel.org/cgit/linux/kernel/git/clk/linux.git/log/?h=clk-sunxi-ng

> On 26.7.2016 08:32, Maxime Ripard wrote:
> > On Thu, Jul 21, 2016 at 11:52:15AM +0200, Ondřej Jirman wrote:
> >>>>> 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.
> >>>>
> >>>> It would need to be tested. U-boot does the change only once, while the
> >>>> kernel would be doing it all the time and between various frequencies
> >>>> and PLL settings. So the issues may show up with this solution too.
> >>>
> >>> That would have the benefit of being quite easy to document, not be a
> >>> huge amount of code and it would work on all the CPUs PLLs we have so
> >>> far, so still, a pretty big win. If it doesn't, of course, we don't
> >>> really have the choice.
> >>
> >> It's probably more code though. It has to access different register from
> >> the one that is already defined in dts, which would add a lot of code
> >> and require dts changes. The original patch I sent is simpler than that.
> > 
> > Why?
> > 
> > You can use container_of to retrieve the parent structure of the clock
> > notifier, and then you get a ccu_common structure pointer, with the
> > CCU base address, the clock register, its lock, etc.
> > 
> > Look at what is done in drivers/clk/meson/clk-cpu.c. It's like 20 LoC.
> > 
> > I don't really get why anything should be changed in the DT, or why it
> > would add a lot of code. Or maybe we're not talking about the same
> > thing?
> 
> I've looked at the new CCU code, particularly ccu_nkmp.c, and found that
> it very liberally uses divider parameters, even in situations that are
> out of spec compared to the current code in the kernel.
> 
> In the current code and especially in the original vendor code, divider
> parameters are used as last resort only. Presumably because, of the
> inherent trouble in changing them, as I described to you in other email.
> 
> The new ccu code uses dividers often and even at very high frequencies,
> which goes against the spec.
> 
> In the vendor code M is never anything else but 0, and P is used only
> for frequencies below 288MHz, which matches the H3 datasheet, which says:

In the vendor code, P is never used either. All the boards we had so
far don't go that low, so we cannot make any of these assumptions,
especially since the vendor code has had the bad habit of doing
something wrong and / or useless in the past.

However, this implementation is a new thing, and it was using the H3
precisely because of its early stage of support to use it as a testbed
for the more established.

If you feel like we should use a different formula to favour the
multipliers over the dividers (or want to change the class of the CPU
PLL to an NKM or something else, this is totally doable.

> "The P factor only use in the condition that PLL output less than 288
> MHz."

And the datasheet also had some issues, either misleading or wrong
comments in the past. Don't get me wrong, I'm not saying that this is
wrong, just that we should not follow it religiously, and that we
should trust more the experiments than the datasheet.

> Also other datasheets of similar socs from Allwinner state that M should
> not be used in production code.

Which ones specifically?

> So it may be that they either forgot to state it in the H3
> datasheet, or it can be used. In any case, they never use M in their
> code, so it may be wise to keep to that.
> 
> When I boot with the new CCU code I get this:
> 
> PLL_CPUX = 0x00001B21 : M = 2, K = 3, N = 28, P = 1, EN = 0
> PLL_CPUX : freq = 1008MHz
> 
> Mathematically it works, but it is against the spec.
> 
> Also, this:
> 
> analyzing CPU 0:
>   driver: cpufreq-dt
>   CPUs which run at the same hardware frequency: 0 1 2 3
>   CPUs which need to have their frequency coordinated by software: 0 1 2 3
>   maximum transition latency: 1.74 ms
>   hardware limits: 120 MHz - 1.37 GHz
>   available frequency steps:  120 MHz, 240 MHz, 480 MHz, 648 MHz, 816
> MHz, 960 MHz, 1.01 GHz, 1.06 GHz, 1.10 GHz, 1.15 GHz, 1.20 GHz, 1.22
> GHz, 1.25 GHz, 1.30 GHz, 1.34 GHz, 1.37 GHz
>   available cpufreq governors: conservative ondemand userspace powersave
> performance
>   current policy: frequency should be within 240 MHz and 240 MHz.
>                   The governor "performance" may decide which speed to use
>                   within this range.
>   current CPU frequency: 24.0 MHz (asserted by call to hardware)
> 
> 
> Somehow, the new CCU code sets the CPUX to 24MHz no matter what.
> 
> I'm using your pen/clk-rework branch without any other patches that were
> later sent to the mailing list.

It's probably because of CLK_SET_RATE_PARENT on the CPU clock missing,
as Chen-Yu suggested.

Maxime

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