netdev - Re: [PATCH] Use a spinlock to guard `fep->ptp_clk

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Message-ID: <79e46d59-436c-ca82-cad4-15c3cb13b1cf@prolan.hu>
Date:   Wed, 31 Aug 2022 16:21:47 +0200
From:   Csókás Bence <csokas.bence@...lan.hu>
To:     Andrew Lunn <andrew@...n.ch>
CC:     <netdev@...r.kernel.org>,
        Richard Cochran <richardcochran@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, <kernel@...gutronix.de>,
        Marc Kleine-Budde <mkl@...gutronix.de>
Subject: Re: [PATCH] Use a spinlock to guard `fep->ptp_clk_on`


On 2022. 08. 31. 16:03, Andrew Lunn wrote:
>> diff --git a/drivers/net/ethernet/freescale/fec_main.c b/drivers/net/ethernet/freescale/fec_main.c
>> index b0d60f898249..98d8f8d6034e 100644
>> --- a/drivers/net/ethernet/freescale/fec_main.c
>> +++ b/drivers/net/ethernet/freescale/fec_main.c
>> @@ -2029,6 +2029,7 @@ static int fec_enet_clk_enable(struct net_device *ndev, bool enable)
>>   {
>>   	struct fec_enet_private *fep = netdev_priv(ndev);
>>   	int ret;
>> +	unsigned long flags;
> 
> Please keep to reverse christmas tree

checkpatch didn't tell me that was a requirement... Easy to fix though.

>    
>>   	if (enable) {
>>   		ret = clk_prepare_enable(fep->clk_enet_out);
>> @@ -2036,15 +2037,15 @@ static int fec_enet_clk_enable(struct net_device *ndev, bool enable)
>>   			return ret;
>>   
>>   		if (fep->clk_ptp) {
>> -			mutex_lock(&fep->ptp_clk_mutex);
>> +			spin_lock_irqsave(&fep->ptp_clk_lock, flags);
> 
> Is the ptp hardware accessed in interrupt context? If not, you can use
> a plain spinlock, not _irqsave..

`fec_suspend()` calls `fec_enet_clk_enable()`, which may be a 
non-preemptible context, I'm not sure how the PM subsystem's internals 
work...
Besides, with the way this driver is built, function call dependencies 
all over the place, I think it's better safe than sorry. I don't think 
there is any disadvantage (besides maybe a few lost cycles) of using 
_irqsave in regular process context anyways.

Bence