linux-kernel - Re: [PATCH 3/3] phy: sun4i-usb: Use power efficient workqueue for debounce and poll

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Message-ID: <971979a1-a69e-c3fe-6799-805407315885@sholland.org>
Date:   Mon, 16 Nov 2020 22:48:37 -0600
From:   Samuel Holland <samuel@...lland.org>
To:     Maxime Ripard <maxime@...no.tech>
Cc:     Frank Lee <frank@...winnertech.com>, vkoul@...nel.org,
        wens@...e.org, krzk@...nel.org, colin.king@...onical.com,
        tiny.windzz@...il.com, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 3/3] phy: sun4i-usb: Use power efficient workqueue for
 debounce and poll

On 11/12/20 3:53 AM, Maxime Ripard wrote:
> On Tue, Nov 10, 2020 at 09:44:37PM -0600, Samuel Holland wrote:
>> On 11/9/20 6:12 AM, Frank Lee wrote:
>>> From: Yangtao Li <frank@...winnertech.com>
>>>
>>> The debounce and poll time is generally quite long and the work not
>>> performance critical so allow the scheduler to run the work anywhere
>>> rather than in the normal per-CPU workqueue.
>>>
>>> Signed-off-by: Yangtao Li <frank@...winnertech.com>
>>> ---
>>>  drivers/phy/allwinner/phy-sun4i-usb.c | 14 +++++++-------
>>>  1 file changed, 7 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/drivers/phy/allwinner/phy-sun4i-usb.c b/drivers/phy/allwinner/phy-sun4i-usb.c
>>> index 651d5e2a25ce..4787ad13b255 100644
>>> --- a/drivers/phy/allwinner/phy-sun4i-usb.c
>>> +++ b/drivers/phy/allwinner/phy-sun4i-usb.c
>>> @@ -326,7 +326,7 @@ static int sun4i_usb_phy_init(struct phy *_phy)
>>>  		/* Force ISCR and cable state updates */
>>>  		data->id_det = -1;
>>>  		data->vbus_det = -1;
>>> -		queue_delayed_work(system_wq, &data->detect, 0);
>>> +		queue_delayed_work(system_power_efficient_wq, &data->detect, 0);
>>>  	}
>>>  
>>>  	return 0;
>>> @@ -444,7 +444,7 @@ static int sun4i_usb_phy_power_on(struct phy *_phy)
>>>  
>>>  	/* We must report Vbus high within OTG_TIME_A_WAIT_VRISE msec. */
>>
>> This doesn't sound like "not performance critical" to me. My understanding is
>> the debouncing has a deadline from the USB spec. Maybe this is more flexible
>> than the comment makes it sound?
> 
> It's not really clear to me what the power_efficient workqueue brings to
> the table exactly from the comments on WQ_POWER_EFFICIENT (and the
> associated gmane link is long dead).
> 
> It's only effect seems to be that it sets WQ_UNBOUND when the proper
> command line option is set, and WQ_UNBOUND allows for the scheduled work
> to run on any CPU instead of the local one.
> 
> Given that we don't have any constraint on the CPU here, and the CPU
> locality shouldn't really make any difference, I'm not sure we should
> expect any meaningful difference.
> 
> This is also what the rest of the similar drivers seem to be using:
> https://elixir.bootlin.com/linux/v5.10-rc3/source/drivers/usb/common/usb-conn-gpio.c#L119
> https://elixir.bootlin.com/linux/v5.10-rc3/source/drivers/usb/core/hub.c#L1254

Thanks for the explanation. Then the patch looks fine to me.

Reviewed-by: Samuel Holland <samuel@...lland.org>

Cheers,
Samuel