From: Benjamin Herrenschmidt <benh@kernel•crashing.org>
To: Linus Torvalds <torvalds@linux-foundation•org>
Cc: Alexander Duyck <alexander.duyck@gmail•com>,
Will Deacon <will.deacon@arm•com>,
Sinan Kaya <okaya@codeaurora•org>, Arnd Bergmann <arnd@arndb•de>,
Jason Gunthorpe <jgg@ziepe•ca>,
David Laight <David.Laight@aculab•com>, Oliver <oohall@gmail•com>,
"open list:LINUX FOR POWERPC (32-BIT AND 64-BIT)"
<linuxppc-dev@lists•ozlabs.org>,
"linux-rdma@vger•kernel.org" <linux-rdma@vger•kernel.org>,
"Paul E. McKenney" <paulmck@linux•vnet.ibm.com>,
"netdev@vger•kernel.org" <netdev@vger•kernel.org>
Subject: Re: RFC on writel and writel_relaxed
Date: Wed, 28 Mar 2018 17:56:05 +1100 [thread overview]
Message-ID: <1522220165.7364.110.camel@kernel.crashing.org> (raw)
In-Reply-To: <CA+55aFyaZ=WEXFg4KZ0yCWaR=8Dm_FPK-xcA0EYi3fEzm+KZkA@mail.gmail.com>
On Wed, 2018-03-28 at 06:53 +0000, Linus Torvalds wrote:
>
>
> On Tue, Mar 27, 2018, 20:43 Benjamin Herrenschmidt <benh@kernel•crash
> ing.org> wrote:
> > >
> > > Of course, you'd have to be pretty odd to want to start a DMA
> > with a
> > > read anyway - partly exactly because it's bad for performance
> > since
> > > reads will be synchronous and not buffered like a write).
> >
> > I have bad memories of old adaptec controllers ...
>
> *Old* adaptec controllers were likely to use the in/out instructions
> for status and command data.
>
> Those are actually even more ordered than UC reads and writes: the
> in/out instructions are not just fully ordered, but are fully
> *synchronous* on x86.
>
> So not just doing accesses in order, but actually waiting for
> everything to drain before they start executing, but they also wait
> for the operation itself to complete (ie "out" will not just queue
> the write, it will then wait for the queue to empty and the write
> data to hit the line).
>
> That's why in/out were *so* slow, and why nobody uses them any more
> (well, the address size limitations and the lack of any remapping of
> the address obviously also are a reason).
All true indeed, though a lot of other archs never quite made them
fully synchronous, which was another can of worms ... oh well.
As for Adaptec, you might be right, I do remember having cases of old
stuff triggering DMA on reads, it might have been "Mac" variants of
Adaptec using MMIO or something...
Cheers,
Ben.
next prev parent reply other threads:[~2018-03-28 6:56 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <1521692689.16434.293.camel@kernel.crashing.org>
[not found] ` <CAOSf1CFv1HHCL3YOVhRn2U=grdNjaQ=A4m3xwxN2Rek1-_TySg@mail.gmail.com>
[not found] ` <1521726722.16434.312.camel@kernel.crashing.org>
[not found] ` <20180323163510.GC13033@ziepe.ca>
[not found] ` <1521854626.16434.359.camel@kernel.crashing.org>
[not found] ` <58ce5b83f40f4775bec1be8db66adb0d@AcuMS.aculab.com>
[not found] ` <20180326165425.GA15554@ziepe.ca>
[not found] ` <CAK8P3a1zeMyj+Z-y4ER4moY6Zip9EWNOinf+VnboGOrgiwbBZA@mail.gmail.com>
[not found] ` <20180326202545.GB15554@ziepe.ca>
[not found] ` <CAK8P3a3fc43ZcW626hmsd3DVcLw7hGkdUMxp7s4Rn3mdkziwMQ@mail.gmail.com>
[not found] ` <20180326210951.GD15554@ziepe.ca>
[not found] ` <CAK8P3a2UU1xAM0NLo7Q4-Xgo1SzY3De1uqpFudr+2ZW7nHEPmA@mail.gmail.com>
[not found] ` <1522101616.7364.13.camel@kernel.crashing.org>
2018-03-27 14:46 ` RFC on writel and writel_relaxed Sinan Kaya
2018-03-27 15:01 ` Jose Abreu
2018-03-27 15:10 ` Will Deacon
2018-03-27 18:54 ` Alexander Duyck
2018-03-27 19:54 ` Arnd Bergmann
2018-03-27 20:46 ` Arnd Bergmann
2018-03-27 21:33 ` Benjamin Herrenschmidt
2018-03-28 0:39 ` Linus Torvalds
2018-03-28 1:03 ` Benjamin Herrenschmidt
2018-03-28 2:51 ` Linus Torvalds
2018-03-28 3:24 ` Sinan Kaya
2018-03-28 4:41 ` Benjamin Herrenschmidt
2018-03-28 6:14 ` Linus Torvalds
2018-03-28 11:41 ` okaya
2018-03-28 15:13 ` Benjamin Herrenschmidt
2018-03-28 15:55 ` David Miller
2018-03-28 16:23 ` Nicholas Piggin
2018-03-28 21:31 ` Benjamin Herrenschmidt
2018-03-28 22:09 ` Nicholas Piggin
2018-03-29 9:20 ` Will Deacon
2018-03-29 13:56 ` Sinan Kaya
2018-03-29 14:04 ` David Miller
2018-03-29 16:29 ` Arnd Bergmann
2018-03-29 16:59 ` Sinan Kaya
2018-03-30 1:40 ` Benjamin Herrenschmidt
2018-04-02 13:01 ` Sinan Kaya
2018-03-28 4:33 ` Benjamin Herrenschmidt
2018-03-28 6:26 ` Linus Torvalds
2018-03-28 6:42 ` Benjamin Herrenschmidt
2018-03-28 6:53 ` Linus Torvalds
2018-03-28 6:56 ` Benjamin Herrenschmidt [this message]
2018-03-28 7:11 ` Arnd Bergmann
2018-03-28 7:42 ` Benjamin Herrenschmidt
2018-03-28 9:07 ` Will Deacon
2018-03-28 9:56 ` Benjamin Herrenschmidt
2018-03-28 10:13 ` Aw: " Lino Sanfilippo
2018-03-28 10:20 ` Benjamin Herrenschmidt
2018-03-28 11:30 ` David Laight
2018-03-28 15:12 ` Benjamin Herrenschmidt
2018-03-28 16:16 ` David Laight
2018-03-28 1:21 ` Benjamin Herrenschmidt
2018-03-27 21:35 ` Benjamin Herrenschmidt
2018-03-27 21:54 Alexander Duyck
2018-03-27 22:35 ` Sinan Kaya
2018-03-27 23:43 ` Benjamin Herrenschmidt
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