netdev - Re: Aw: Re: RFC on writel and writel

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Message-ID: <1522232416.21446.10.camel@kernel.crashing.org>
Date:   Wed, 28 Mar 2018 21:20:16 +1100
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Lino Sanfilippo <LinoSanfilippo@....de>
Cc:     Will Deacon <will.deacon@....com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Sinan Kaya <okaya@...eaurora.org>,
        Arnd Bergmann <arnd@...db.de>, Jason Gunthorpe <jgg@...pe.ca>,
        David Laight <David.Laight@...lab.com>,
        Oliver <oohall@...il.com>,
        "open list:LINUX FOR POWERPC (32-BIT AND 64-BIT)" 
        <linuxppc-dev@...ts.ozlabs.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: Aw: Re: RFC on writel and writel_relaxed

On Wed, 2018-03-28 at 12:13 +0200, Lino Sanfilippo wrote:
> Hi,
> 
> 
> > 
> > Yeah so that other trick I'm talking about is also used for timing
> > accuracy.
> > 
> > For example, let's say I have a device with a reset bit and the spec
> > says the reset bit needs to be set for at least 10us.
> > 
> > This is wrong:
> > 
> > 	writel(1, RESET_REG);
> > 	usleep(10);
> > 	writel(0, RESET_REG);
> > 
> > Because of write posting, the first write might arrive to the device
> > right before the second one.
> > 
> 
> Does not write posting only concern PCI? This seems to be a different topic. Furthermore
> write posting should not include write reordering...

Nobody's talking about re-ordering and no, write posting is rather
common practice on a whole lot of different busses, not just PCI(e).

Cheers,
Ben.