linux-kernel - Re: [PATCH 2/2] clk: bcm2835: Skip PLLC clocks when deciding on a new clock parent

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Date:	Sat, 30 Apr 2016 11:28:25 +0200
From:	Martin Sperl <kernel@...tin.sperl.org>
To:	Eric Anholt <eric@...olt.net>
Cc:	Michael Turquette <mturquette@...libre.com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	linux-rpi-kernel@...ts.infradead.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Stephen Warren <swarren@...dotorg.org>,
	Lee Jones <lee@...nel.org>
Subject: Re: [PATCH 2/2] clk: bcm2835: Skip PLLC clocks when deciding on a new clock parent


> On 26.04.2016, at 21:39, Eric Anholt <eric@...olt.net> wrote:
> 
> If the firmware had set up a clock to source from PLLC, go along with
> it.  But if we're looking for a new parent, we don't want to switch it
> to PLLC because the firmware will force PLLC (and thus the AXI bus
> clock) to different frequencies during over-temp/under-voltage,
> without notification to Linux.
> 
> On my system, this moves the Linux-enabled HDMI state machine and DSI1
> escape clock over to plld_per from pllc_per.  EMMC still ends up on
> pllc_per, because the firmware had set it up to use that.
> 
> Signed-off-by: Eric Anholt <eric@...olt.net>
> Fixes: 41691b8862e2 ("clk: bcm2835: Add support for programming the audio domain clocks")
> —

I guess this patch looks to me as if it is a policy inside the kernel,
which is AFAIK frowned upon.

I am looking into making "assigned-clock-parents” inside the dt 
work with the driver.

Could look something like this:
i2s: i2s@...03000 {
	assigned-clock-parents = <&cprman BCM2835_PLLD_PER>, <&clk_osc>;
	assigned-clocks = <&cprman BCM2835_CLOCK_PCM>, <&cprman BCM2835_CLOCK_PCM>;
};
(not sure if that works really - the same clock in assigned-clocks looks suspicious)

This would move the policy out of the kernel into the device-tree,
which - i guess is a better solution.

Martin