linux-kernel - Re: [PATCH 1/3] clk: sunxi-ng: Add clk notifier to gate then ungate PLL clocks

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Date:   Thu, 13 Apr 2017 09:02:09 +0200
From:   Maxime Ripard <maxime.ripard@...e-electrons.com>
To:     Chen-Yu Tsai <wens@...e.org>
Cc:     Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...eaurora.org>,
        linux-arm-kernel@...ts.infradead.org, linux-clk@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-sunxi@...glegroups.com
Subject: Re: [PATCH 1/3] clk: sunxi-ng: Add clk notifier to gate then ungate
 PLL clocks

Hi Chen-Yu,

On Thu, Apr 13, 2017 at 10:13:52AM +0800, Chen-Yu Tsai wrote:
> In common PLL designs, changes to the dividers take effect almost
> immediately, while changes to the multipliers (implemented as
> dividers in the feedback loop) take a few cycles to work into
> the feedback loop for the PLL to stablize.
> 
> Sometimes when the PLL clock rate is changed, the decrease in the
> divider is too much for the decrease in the multiplier to catch up.
> The PLL clock rate will spike, and in some cases, might lock up
> completely. This is especially the case if the divider changed is
> the pre-divider, which affects the reference frequency.
> 
> This patch introduces a clk notifier callback that will gate and
> then ungate a clk after a rate change, effectively resetting it,
> so it continues to work, despite any possible lockups. Care must
> be taken to reparent any consumers to other temporary clocks during
> the rate change, and that this notifier callback must be the first
> to be registered.
> 
> This is intended to fix occasional lockups with cpufreq on newer
> Allwinner SoCs, such as the A33 and the H3. Previously it was
> thought that reparenting the cpu clock away from the PLL while
> it stabilized was enough, as this worked quite well on the A31.
> 
> On the A33, hangs have been observed after cpufreq was recently
> introduced. With the H3, a more thorough test [1] showed that
> reparenting alone isn't enough. The system still locks up unless
> the dividers are limited to 1.
> 
> A hunch was if the PLL was stuck in some unknown state, perhaps
> gating then ungating it would bring it back to normal. Tests
> done by Icenowy Zheng using Ondrej's test firmware shows this
> to be a valid solution.
> 
> [1] http://www.spinics.net/lists/arm-kernel/msg552501.html
> 
> Reported-by: Ondrej Jirman <megous@...ous.com>
> Signed-off-by: Chen-Yu Tsai <wens@...e.org>
> Tested-by: Icenowy Zheng <icenowy@...c.io>
> Tested-by: Quentin Schulz <quentin.schulz@...e-electrons.com>

Thanks for looking into this, and coming up with a clean solution, and
a great commit log.

However, I wondering, isn't that notifier just a re-implementation of
CLK_SET_RATE_GATE?

Maxime

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

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