linux-kernel - Re: [PATCH v2 4/8] clk: migrate the count of orphaned clocks at init

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Date:   Sun, 11 Mar 2018 22:54:08 -0700
From:   Michael Turquette <mturquette@...libre.com>
To:     Heiko Stübner <heiko@...ech.de>,
        Jerome Brunet <jbrunet@...libre.com>
Cc:     Dong Aisheng <aisheng.dong@....com>, linux-clk@...r.kernel.org,
        linux-kernel@...r.kernel.org, Stephen Boyd <sboyd@...eaurora.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Shawn Guo <shawnguo@...nel.org>
Subject: Re: [PATCH v2 4/8] clk: migrate the count of orphaned clocks at init

Excerpts from Heiko Stübner's message of February 15, 2018 1:01 pm:
> Am Mittwoch, 14. Februar 2018, 14:43:36 CET schrieb Jerome Brunet:
>> The orphan clocks reparents should migrate any existing count from the
>> orphan clock to its new acestor clocks, otherwise we may have
>> inconsistent counts in the tree and end-up with gated critical clocks
>> 
>> Assuming we have two clocks, A and B.
>> * Clock A has CLK_IS_CRITICAL flag set.
>> * Clock B is an ancestor of A which can gate. Clock B gate is left
>>   enabled by the bootloader.
>> 
>> Step 1: Clock A is registered. Since it is a critical clock, it is
>> enabled. The clock being still an orphan, no parent are enabled.
>> 
>> Step 2: Clock B is registered and reparented to clock A (potentially
>> through several other clocks). We are now in situation where the enable
>> count of clock A is 1 while the enable count of its ancestors is 0, which
>> is not good.
>> 
>> Step 3: in lateinit, clk_disable_unused() is called, the enable_count of
>> clock B being 0, clock B is gated and and critical clock A actually gets
>> disabled.
>> 
>> This situation was found while adding fdiv_clk gates to the meson8b
>> platform.  These clocks parent clk81 critical clock, which is the mother
>> of all peripheral clocks in this system. Because of the issue described
>> here, the system is crashing when clk_disable_unused() is called.
>> 
>> The situation is solved by reverting
>> commit f8f8f1d04494 ("clk: Don't touch hardware when reparenting during
>> registration"). To avoid breaking again the situation described in this
>> commit
>> description, enabling critical clock should be done before walking the
>> orphan list. This way, a parent critical clock may not be accidentally
>> disabled due to the CLK_OPS_PARENT_ENABLE mechanism.
>> 
>> Fixes: f8f8f1d04494 ("clk: Don't touch hardware when reparenting during
>> registration") Cc: Stephen Boyd <sboyd@...eaurora.org>
>> Cc: Shawn Guo <shawnguo@...nel.org>
>> Cc: Dong Aisheng <aisheng.dong@....com>
>> Signed-off-by: Jerome Brunet <jbrunet@...libre.com>
> 
> On a rk3288-veyron Chromebook
> Tested-by: Heiko Stuebner <heiko@...ech.de>
> 
> This made the hdmi on the machine work again with 4.16-rc1 .
> (hdmi-cec clock is sourced from the i2c connected pmic which provides
> the 32kHz clock needed)
> 
> So it would be really cool if this could make it into 4.16-rc :-)

I separated this patch out into clk-fixes to be set for -rc5 or -rc6.

Regards,
Mike

> 
> 
> Heiko
>