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

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Date:   Fri, 29 Sep 2017 09:14:26 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Andrew Lunn' <andrew@...n.ch>,
        "Tristram.Ha@...rochip.com" <Tristram.Ha@...rochip.com>
CC:     "muvarov@...il.com" <muvarov@...il.com>,
        "pavel@....cz" <pavel@....cz>,
        "nathan.leigh.conrad@...il.com" <nathan.leigh.conrad@...il.com>,
        "vivien.didelot@...oirfairelinux.com" 
        <vivien.didelot@...oirfairelinux.com>,
        "f.fainelli@...il.com" <f.fainelli@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Woojung.Huh@...rochip.com" <Woojung.Huh@...rochip.com>
Subject: RE: [PATCH RFC 3/5] Add KSZ8795 switch driver

From: Andrew Lunn
> Sent: 28 September 2017 20:34
...
> > 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.

Is the SPI access software bit-banged?
Doing that with software delays isn't friendly to the rest of the system.
(Hardware guys please note...)

One possibility is to rate-limit the stats reading.
Then an application cannot completely 'hog' the SPI bandwidth.

	David