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

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Date:   Wed, 18 Nov 2020 19:00:44 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Claudiu Manoil <claudiu.manoil@....com>,
        "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

On Wed, Nov 18, 2020 at 02:38:56PM +0100, Andrew Lunn wrote:
> On Tue, Nov 17, 2020 at 10:22:20AM +0000, Claudiu Manoil wrote:
> > >-----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.
> 
> Hi CLaidiu
> 
> Thanks for the explanation. I don't think i've every reviewed a driver
> using read/write locks like this. But thinking it through, it does
> seem O.K.

Thanks for reviewing and getting this merged. It sure is helpful to not
have the link flap while running iperf3 or other intensive network
activity.

Even if this use of rwlocks may seem unconventional, I think it is the
right tool for working around the hardware bug.