linux-kernel - Re: [PATCH 3/3] clk: bcm2835: Skip PLLC clocks when deciding on a new clock parent

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Date:	Wed, 11 May 2016 15:58:54 -0700
From:	Stephen Boyd <sboyd@...eaurora.org>
To:	Eric Anholt <eric@...olt.net>
Cc:	Michael Turquette <mturquette@...libre.com>,
	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 3/3] clk: bcm2835: Skip PLLC clocks when deciding on a
 new clock parent

On 05/09, Eric Anholt 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.

Is it ok for EMMC rate to change with over-temp/under-voltage?
The description makes it sound like PLLC is for clks that want to
run at some system bus rate and they don't care about exact
frequencies.

> 
> Signed-off-by: Eric Anholt <eric@...olt.net>
> Fixes: 41691b8862e2 ("clk: bcm2835: Add support for programming the audio domain clocks")
> diff --git a/drivers/clk/bcm/clk-bcm2835.c b/drivers/clk/bcm/clk-bcm2835.c
> index 1091012ecec6..1d8f29ea9f69 100644
> --- a/drivers/clk/bcm/clk-bcm2835.c
> +++ b/drivers/clk/bcm/clk-bcm2835.c
> @@ -1008,16 +1008,28 @@ static int bcm2835_clock_set_rate(struct clk_hw *hw,
>  	return 0;
>  }
>  
> +static bool
> +bcm2835_clk_is_pllc(struct clk_hw *hw)
> +{
> +	if (!hw)
> +		return false;
> +
> +	return strncmp(clk_hw_get_name(hw), "pllc", 4) == 0;

This strcmp is not great. Any chance we could look for the parent
by reading the hardware and knowing what bit corresponds to pllc
as a parent? That would be much nicer so that we don't rely on
string comparisons for something the hardware can tell us.

-- 
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a Linux Foundation Collaborative Project