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

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Message-ID: <20170929203926.GD17713@lunn.ch>
Date:   Fri, 29 Sep 2017 22:39:26 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Tristram.Ha@...rochip.com
Cc:     David.Laight@...LAB.COM, 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 Fri, Sep 29, 2017 at 07:19:17PM +0000, Tristram.Ha@...rochip.com wrote:
> > > My concern is if a task is already running with SPI access to a lot
> > > of registers like reading the 32 MIB counters in every port of the
> > > switch, another register access has to wait until they are finished.
> > 
> > Why does it have to wait? Looking at the code in
> > ksz_get_ethtool_stats(), you don't take any mutex which will prevent
> > others from using the SPI bus. All there is is a mutex which prevents
> > two sets of ksz_get_ethtool_stats() at the same time.
> > 
> > So a PTP read could happen in parallel, and will not be blocked by MIB
> > reads. They should get interleaved access to the SPI bus.
> > 
> 
> The MIB counters are read in the background.  For multiple CPU cores 2
> tasks may run in the same time allowing SPI access one after another.
> For single core I am not sure an SPI access like coming from an interrupt
> routine can jump ahead from one in a background task.

The SPI subsystem has a mutex per controller. When starting a
transfer, it takes the mutex and release it once the transfer has
completed. There is also a reschedule point at the end of a
transfer. So even on your single core CPU, there can be multi tasking
going on.

      Andrew