netdev - RE: [PATCH net] enetc: Workaround for MDIO register access issue

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Date:   Tue, 17 Nov 2020 10:22:20 +0000
From:   Claudiu Manoil <claudiu.manoil@....com>
To:     Andrew Lunn <andrew@...n.ch>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        "David S . Miller" <davem@...emloft.net>,
        Alexandru Marginean <alexandru.marginean@....com>,
        Vladimir Oltean <vladimir.oltean@....com>
Subject: RE: [PATCH net] enetc: Workaround for MDIO register access issue

>-----Original Message-----
>From: Andrew Lunn <andrew@...n.ch>
>Sent: Tuesday, November 17, 2020 4:45 AM
>To: Claudiu Manoil <claudiu.manoil@....com>
>Cc: netdev@...r.kernel.org; Jakub Kicinski <kuba@...nel.org>; David S .
>Miller <davem@...emloft.net>; Alexandru Marginean
><alexandru.marginean@....com>; Vladimir Oltean
><vladimir.oltean@....com>
>Subject: Re: [PATCH net] enetc: Workaround for MDIO register access issue
>
>> +static inline void enetc_lock_mdio(void)
>> +{
>> +	read_lock(&enetc_mdio_lock);
>> +}
>> +
>
>> +static inline u32 _enetc_rd_mdio_reg_wa(void __iomem *reg)
>> +{
>> +	unsigned long flags;
>> +	u32 val;
>> +
>> +	write_lock_irqsave(&enetc_mdio_lock, flags);
>> +	val = ioread32(reg);
>> +	write_unlock_irqrestore(&enetc_mdio_lock, flags);
>> +
>> +	return val;
>> +}
>
>Can you mix read_lock() with write_lock_irqsave()?  Normal locks you
>should not mix, so i assume read/writes also cannot be mixed?
>

Not sure I understand your concerns, but this is the readers-writers locking
scheme. The readers (read_lock) are "lightweight", they get the most calls,
can be taken from any context including interrupt context, and compete only
with the writers (write_lock). The writers can take the lock only when there are
no readers holding it, and the writer must insure that it doesn't get preempted
(by interrupts etc.) when holding the lock (irqsave). The good part is that mdio
operations are not frequent. Also, we had this code out of the tree for quite some
time, it's well exercised.