[PATCH] soc: mediatek: Fix random hang up issue while kernel init
jamesjj.liao at mediatek.com
Thu Oct 1 20:00:38 PDT 2015
On Thu, 2015-10-01 at 18:08 +0800, Daniel Kurtz wrote:
> I see two cases where "a power domain is a consumer of a clock":
> (a) the clock is needed to access the power domain control
> registers. The clock must actually be in another power domain, since
> otherwise you could never turn *on* the power domain in question.
> (b) the clock is needed to talk to the power domain that is about to
> turn off, or that was just turned on - these clocks are usually used
> to control some kind of bus arbitration, etc. In this case, the
> clocks are only accessed when the power domain is on.
> I would argue that in both cases, the clock should in theory should
> only be enabled/disabled by the power on/off routine for the duration
> of time that is needed, and that it should not be "left enabled" by
> the power domain on/off function... I can see how this might be a
> useful optimization - but it also seems like a way to burn extra power
> by leaving on a clock that is not necessarily being used.
> But - perhaps I am over thinking this, and it is standard practice to
> bundle power domains with the clocks that service components in the
> power domain.
Yes, you are right. Power domain on/off operations need clocks to get
status of the subsystem. Keeping clocks on during power domain on is an
But to model a clock controller as a power domain consumer, or say we
need to control power domain before clock on/off, is not a good practice
because we always consider clock operations should be light weight.
Power domains should be controlled explicitly by subsystem drivers,
instead of hiding behind clock controls.
> Or, alternatively, just prepare venc_sel once during init, for each
> clock controller that needs it, when configuring the clock controller
> node (for example, in mtk_vencsys_init()). This is similar to how we
> set up 'critical clocks'.
> By just preparing the clock, you tell the common clock core:
> "my clock controller will need this clock later; please make sure
> that it can be enabled/disabled later during atomic context."
In current implementation, PLLs will be turned on in clk_prepare(). So
if a clock is always prepared, its parent PLL will be kept on even if
the clock is not enabled. It consumes extra power, so we don't prefer
this kind of solution.
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