linux-kernel - Re: [PATCH v1] phy: qcom-qmp: Raise qcom_qmp_phy

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Date:   Mon, 24 Jun 2019 13:55:57 +0200
From:   Marc Gonzalez <marc.w.gonzalez@...e.fr>
To:     Kishon Vijay Abraham I <kishon@...com>,
        Vivek Gautam <vivek.gautam@...eaurora.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Douglas Anderson <dianders@...omium.org>
Cc:     MSM <linux-arm-msm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1] phy: qcom-qmp: Raise qcom_qmp_phy_enable() polling
 delay

On 20/06/2019 08:25, Kishon Vijay Abraham I wrote:

> On 14/06/19 6:08 PM, Marc Gonzalez wrote:
> 
>> The issue is usleep_range() being misused ^_^
>>
>> Although usleep_range() takes unsigned longs as parameters, it is
>> not appropriate over the entire 0-2^64 range.
>>
>> a) It should not be used with tiny values, because the cost of programming
>> the timer interrupt, and processing the resulting IRQ would dominate.
>>
>> b) It should not be used with large values (above 2000000/HZ) because
>> msleep() is more efficient, and is acceptable for these ranges.
> 
> Documentation/timers/timers-howto.txt has all the information on the various
> kernel delay/sleep mechanisms. For < ~10us, it recommends to use udelay
> (readx_poll_timeout_atomic). Depending on the actual timeout to be used, the
> delay mechanism in timers-howto.txt should be used.

Hello Kishon,

I believe the proposed patch does the right thing:

a) polling for the ready bit is not done in atomic context,
therefore we don't need to busy-loop

b) since we're ultimately calling usleep_range(), we should
pass an appropriate parameter, such as max_us = 10
(instead of max_us = 1, which is outside usleep_range spec)

Maybe it would help if someone reviewed this patch.

Regards.