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由 Chen-Yu Tsai 提交于
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.htmlReported-by: NOndrej Jirman <megous@megous.com> Signed-off-by: NChen-Yu Tsai <wens@csie.org> Tested-by: NIcenowy Zheng <icenowy@aosc.io> Tested-by: NQuentin Schulz <quentin.schulz@free-electrons.com> Signed-off-by: NMaxime Ripard <maxime.ripard@free-electrons.com>
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