linux-kernel - Re: [PATCH RFC 3/5] Add KSZ8795 switch driver

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Message-ID: <20170928193416.GH14940@lunn.ch>
Date:   Thu, 28 Sep 2017 21:34:16 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Tristram.Ha@...rochip.com
Cc:     muvarov@...il.com, pavel@....cz, nathan.leigh.conrad@...il.com,
        vivien.didelot@...oirfairelinux.com, f.fainelli@...il.com,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Woojung.Huh@...rochip.com
Subject: Re: [PATCH RFC 3/5] Add KSZ8795 switch driver

On Mon, Sep 18, 2017 at 08:27:13PM +0000, Tristram.Ha@...rochip.com wrote:
> > > +/**
> > > + * Some counters do not need to be read too often because they are less
> > likely
> > > + * to increase much.
> > > + */
> > 
> > What does comment mean? Are you caching statistics, and updating
> > different values at different rates?
> > 
> 
> There are 34 counters.  In normal case using generic bus I/O or PCI to read them
> is very quick, but the switch is mostly accessed using SPI, or even I2C.  As the SPI
> access is very slow.

How slow is it? The Marvell switches all use MDIO. It is probably a
bit faster than I2C, but it is a lot slower than MMIO or PCI.

ethtool -S lan0 takes about 25ms.

No other driver does caching. So i'm hesitant to add one which does.

>  These accesses can be getting 1588 PTP timestamps and opening/closing ports.

You could drop the mutex between each statistic read, so allowing
something else access to the switch. That should reduce the jitter PTP
experiences.

	Andrew