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

From: Andrew Lunn <[email-masked]>
Date: Thu, 28 Sep 2017 21:34:16 +0200

On Mon, Sep 18, 2017 at 08:27:13PM +0000, [email-masked] 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