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

From <Tristram.Ha@microchip.com>
Newsgroups linux.kernel
Subject RE: [PATCH RFC 3/5] Add KSZ8795 switch driver
Date 2017-09-29 21:20 +0200
Message-ID <uv8Zk-1R9-5@gated-at.bofh.it> (permalink)
References (3 earlier) <uuMP8-4CX-17@gated-at.bofh.it> <uuZCF-4gp-13@gated-at.bofh.it> <uv2qS-642-15@gated-at.bofh.it> <uv8cX-1ku-5@gated-at.bofh.it> <uv8FX-1v8-7@gated-at.bofh.it>
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> > 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.

I know nowadays SoCs are powerful enough to do amazing things.  It is
just I spent a long time using a low-powered SoC doing switch driver
development.

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Thread

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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