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

From	Andrew Lunn <andrew@lunn.ch>
Newsgroups	linux.kernel
Subject	Re: [PATCH RFC 3/5] Add KSZ8795 switch driver
Date	2017-09-28 21:40 +0200
Message-ID	<uuMP8-4CX-17@gated-at.bofh.it> (permalink)
References	<uncnp-5CP-51@gated-at.bofh.it> <undCO-6jj-13@gated-at.bofh.it> <uraQ1-2OE-1@gated-at.bofh.it>
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On Mon, Sep 18, 2017 at 08:27:13PM +0000, Tristram.Ha@microchip.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

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Re: [PATCH RFC 3/5] Add KSZ8795 switch driver Andrew Lunn <andrew@lunn.ch> - 2017-09-28 21:40 +0200
  RE: [PATCH RFC 3/5] Add KSZ8795 switch driver David Laight <David.Laight@ACULAB.COM> - 2017-09-29 11:20 +0200
    Re: [PATCH RFC 3/5] Add KSZ8795 switch driver Andrew Lunn <andrew@lunn.ch> - 2017-09-29 14:20 +0200
      RE: [PATCH RFC 3/5] Add KSZ8795 switch driver <Tristram.Ha@microchip.com> - 2017-09-29 20:30 +0200
        Re: [PATCH RFC 3/5] Add KSZ8795 switch driver Andrew Lunn <andrew@lunn.ch> - 2017-09-29 21:00 +0200
          RE: [PATCH RFC 3/5] Add KSZ8795 switch driver <Tristram.Ha@microchip.com> - 2017-09-29 21:20 +0200
            Re: [PATCH RFC 3/5] Add KSZ8795 switch driver Andrew Lunn <andrew@lunn.ch> - 2017-09-29 22:50 +0200

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