netdev - Re: [PATCH v2] dec: tulip: de4x5: Replace mdelay with usleep_range in de4x5_hw

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Date:   Thu, 12 Apr 2018 07:51:42 +0530
From:   arvindY <arvind.yadav.cs@...il.com>
To:     Jia-Ju Bai <baijiaju1990@...il.com>,
        James Bottomley <James.Bottomley@...senPartnership.com>,
        davem@...emloft.net, stephen@...workplumber.org,
        johannes.berg@...el.com, dhowells@...hat.com
Cc:     netdev@...r.kernel.org, linux-parisc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] dec: tulip: de4x5: Replace mdelay with usleep_range in
 de4x5_hw_init



On Thursday 12 April 2018 07:00 AM, Jia-Ju Bai wrote:
>
>
> On 2018/4/12 0:16, James Bottomley wrote:
>> On Wed, 2018-04-11 at 23:39 +0800, Jia-Ju Bai wrote:
>>> de4x5_hw_init() is never called in atomic context.
>>>
>>> de4x5_hw_init() is only called by de4x5_pci_probe(), which is only
>>> set as ".probe" in struct pci_driver.
>>>
>>> Despite never getting called from atomic context, de4x5_hw_init()
>>> calls mdelay() to busily wait. This is not necessary and can be
>>> replaced with usleep_range() to  avoid busy waiting.
>>>
>>> This is found by a static analysis tool named DCNS written by myself.
>>> And I also manually check it.
>> Did you actually test this?  The usual reason for wanting m/udelay is
>> that the timing must be exact.  The driver is filled with mdelay()s for
>> this reason.  The one you've picked on is in the init path so it won't
>> affect the runtime in any way.  I also don't think we have the hrtimer
>> machinery for usleep_range() to work properly on parisc, so I don't
>> think the replacement works.
>>
>> James
>>
>
> Hello, James.
> Thanks for your reply :)
>
> I agree that usleep_range() here will not much affect the real 
> execution of this driver.
>
> But I think usleep_range() can more opportunity for other threads to 
> use the CPU core to schedule during waiting.
> That is why I detect mdelay() that can be replaced with msleep() or 
> usleep_range().
>

James is right, You have added all usleep_range() during system boot-up 
time.
During boot-up system will run as single threaded. Where this change will
not make much sense. System first priority is match the exact timing on
each and every boot-up.

~arvind

>
> Best wishes,
> Jia-Ju Bai