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

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Message-ID: <df283c24-5a6e-8d17-ca61-10c66ddf4fca@gmail.com>
Date:   Thu, 12 Apr 2018 10:26:47 +0800
From:   Jia-Ju Bai <baijiaju1990@...il.com>
To:     arvindY <arvind.yadav.cs@...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 2018/4/12 10:21, arvindY wrote:
>
>
> 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.
>

Hello, Arvind.
Thanks for your reply :)

I admit I am not familiar with this driver.
I did not know this driver is only loaded during system boot-up time,
I thought this driver can be loaded as a kernel module (like many 
drivers) after system booting.
After knowing this, I admit my patch is not proper, sorry...


Best wishes,
Jia-Ju Bai