linux-kernel - Re: [PATCH review for 4.4 12/47] clk: wm831x: fix usleep

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Date:   Sun, 24 Sep 2017 00:18:12 +0000
From:   "Levin, Alexander (Sasha Levin)" <alexander.levin@...izon.com>
To:     Charles Keepax <ckeepax@...nsource.cirrus.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Nicholas Mc Guire <hofrat@...dl.org>,
        Stephen Boyd <sboyd@...eaurora.org>
Subject: Re: [PATCH review for 4.4 12/47] clk: wm831x: fix usleep_range with
 bad range

On Fri, Sep 22, 2017 at 09:46:28AM +0100, Charles Keepax wrote:
>On Wed, Sep 20, 2017 at 04:45:02AM +0000, Levin, Alexander (Sasha Levin) wrote:
>> From: Nicholas Mc Guire <hofrat@...dl.org>
>>
>> [ Upstream commit ed784c532a3d0959db488f40a96c5127f63d42dc ]
>>
>> The delay here is not in atomic context and does not seem critical with
>> respect to precision, but usleep_range(min,max) with min==max results in
>> giving the timer subsystem no room to optimize uncritical delays. Fix
>> this by setting the range to 2000,3000 us.
>>
>> Fixes: commit f05259a6ffa4 ("clk: wm831x: Add initial WM831x clock driver")
>> Signed-off-by: Nicholas Mc Guire <hofrat@...dl.org>
>> Acked-by: Charles Keepax <ckeepax@...nsource.wolfsonmicro.com>
>> Signed-off-by: Stephen Boyd <sboyd@...eaurora.org>
>> Signed-off-by: Sasha Levin <alexander.levin@...izon.com>
>> ---
>>  drivers/clk/clk-wm831x.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/clk/clk-wm831x.c b/drivers/clk/clk-wm831x.c
>> index 763aed2de893..dfedcf5bc429 100644
>> --- a/drivers/clk/clk-wm831x.c
>> +++ b/drivers/clk/clk-wm831x.c
>> @@ -101,7 +101,8 @@ static int wm831x_fll_prepare(struct clk_hw *hw)
>>  	if (ret != 0)
>>  		dev_crit(wm831x->dev, "Failed to enable FLL: %d\n", ret);
>>
>> -	usleep_range(2000, 2000);
>> +	/* wait 2-3 ms for new frequency taking effect */
>> +	usleep_range(2000, 3000);
>
>Does this patch really make sense for stable, isn't this really
>just a small optimisation? The patch is pretty harmless so I
>can't see applying it causing any problems, just curious what
>problems not having it is causing.

Looking back at this, I think I misunderstood a scenario in the scheduler this might be causing. What you say makes sense, I'll drop it.

-- 

Thanks,
Sasha