netdev - Re: [PATCH net-next 1/5] net: phy: mscc: add ethtool statistics counters

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Message-ID: <20180914132959.GH14865@lunn.ch>
Date:   Fri, 14 Sep 2018 15:29:59 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Quentin Schulz <quentin.schulz@...tlin.com>
Cc:     davem@...emloft.net, f.fainelli@...il.com,
        allan.nielsen@...rochip.com, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, thomas.petazzoni@...tlin.com,
        Raju Lakkaraju <Raju.Lakkaraju@...rosemi.com>
Subject: Re: [PATCH net-next 1/5] net: phy: mscc: add ethtool statistics
 counters

> When you change a page, you basically can access only the registers in
> this page so if there are two functions requesting different pages at
> the same time or registers of different pages, it won't work well
> indeed.
> 
> > phy_read_page() and phy_write_page() will do the needed locking if
> > this is an issue.
> > 
> 
> That's awesome! Didn't know it existed. Thanks a ton!
> 
> Well, that means I should migrate the whole driver to use
> phy_read/write_paged instead of the phy_read/write that is currently in
> use.
> 
> That's impacting performance though as per phy_read/write_paged we read
> the current page, set the desired page, read/write the register, set the
> old page back. That's 4 times more operations.

You can use the lower level locking primatives. See m88e1318_set_wol()
for example.

> Couldn't we use the
> phy_device mutex instead (as it's currently done in the whole driver)?
> Or is it worse/comparable in performance to the suggested solution?

Russell King found a race condition where this breaks. You cannot hold
the phy_device mutex everywhere.

    Andrew