linux-kernel - Re: [PATCH v2] clk: exynos5420: Keep aclk66

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Date:	Thu, 05 Jun 2014 21:31:53 +0200
From:	Tomasz Figa <tomasz.figa@...il.com>
To:	Doug Anderson <dianders@...omium.org>
CC:	Tomasz Figa <t.figa@...sung.com>,
	Mike Turquette <mturquette@...aro.org>,
	Kukjin Kim <kgene.kim@...sung.com>,
	linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Olof Johansson <olof@...om.net>,
	Javier Martinez Canillas <javier.martinez@...labora.co.uk>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2] clk: exynos5420: Keep aclk66_peric enabled during
 boot

On 05.06.2014 21:22, Doug Anderson wrote:
> Tomasz,
> 
> On Thu, Jun 5, 2014 at 12:14 PM, Tomasz Figa <tomasz.figa@...il.com> wrote:
>> On 05.06.2014 20:48, Doug Anderson wrote:
>>> Tomasz / Mike,
>>>
>>> On Fri, May 30, 2014 at 9:32 AM, Doug Anderson <dianders@...omium.org> wrote:
>>>> Right now if you've got earlyprintk enabled on exynos5420-peach-pit
>>>> then you'll get a hang on boot.  Here's why:
>>>>
>>>> 1. The i2c-s3c2410 driver will probe at subsys_initcall.  It will
>>>>    enable its clock and disable it.  This is the clock "i2c2".
>>>> 2. The act of disabling "i2c2" will disable its parents.  In this case
>>>>    the parent is "aclk66_peric".  There are no other children of
>>>>    "aclk66_peric" officially enabled, so "aclk66_peric" will be turned
>>>>    off (despite being CLK_IGNORE_UNUSED, but that's by design).
>>>> 3. The next time you try to earlyprintk you'll do so without the UART
>>>>    clock enabled.  That's because the UART clocks are also children of
>>>>    "aclk66_peric".  You'll hang.
>>>>
>>>> There's no good place to put a clock enable for earlyprintk, which is
>>>> handled by a bunch of assembly code.  The best we can do is to handle
>>>> this in the clock driver.
>>>>
>>>> Signed-off-by: Doug Anderson <dianders@...omium.org>
>>>> Tested-by: Javier Martinez Canillas <javier.martinez@...labora.co.uk>
>>>> ---
>>>> Changes in v2:
>>>> - Use GATE_A and clk_get().  Save the clock for putting later.
>>>> - Return 0 from exynos5420_clk_late_init().
>>>>
>>>>  drivers/clk/samsung/clk-exynos5420.c | 33 +++++++++++++++++++++++++++++++--
>>>>  1 file changed, 31 insertions(+), 2 deletions(-)
>>>
>>> Are there other changes you'd like me to make to this?  It would be
>>> really nice to get this in for 3.16 so the system doesn't just
>>> mysteriously hang when you use earlyprintk.
>>
>> We can probably take this as an -rc fix, so it should be fine.
>>
>> However I still don't see the point of exporting this clock and
>> polluting the global clkdev namespace. Even the diffstat looks better in v1.
> 
> OK, I'm happy to go back to v1 with the addition of the "return 0".
> 
> Mike: are you OK with that?
> 

One more question that just came to my mind: Why it is just the
aclk66_peri and not its children related to UARTs?

Exynos5420 clock driver still needs a little clean-up, so I'm okay with
any fix for now and changing this later if needed, but is this gate
clock for ACLK66_PERIC even needed? It seems to be marked with
CLK_IGNORE_UNUSED which suggests that it is not needed in this driver at
all, along with other similar clocks. AFAIK for power management
purposes our fine-grained approach is fine and we shouldn't need to use
the big gates.

Best regards,
Tomasz
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