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>,
Alexander Duyck <alexander.h.duyck@redhat•com>,
"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 15:33:40 +1100 [thread overview]
Message-ID: <1522211620.7364.94.camel@kernel.crashing.org> (raw)
In-Reply-To: <CA+55aFyTeuXPmC5-1JGh2ow-rf5wMA6Vcv0aeS_q0_EnptSK9Q@mail.gmail.com>
On Tue, 2018-03-27 at 16:51 -1000, Linus Torvalds wrote:
> On Tue, Mar 27, 2018 at 3:03 PM, Benjamin Herrenschmidt
> <benh@kernel•crashing.org> wrote:
> >
> > The discussion at hand is about
> >
> > dma_buffer->foo = 1; /* WB */
> > writel(KICK, DMA_KICK_REGISTER); /* UC */
>
> Yes. That certainly is ordered on x86. In fact, afaik it's ordered
> even if that writel() might be of type WC, because that only delays
> writes, it doesn't move them earlier.
Ok so this is our answer ...
... snip ... (thanks for the background info !)
> Oh, the above UC case is absoutely guaranteed.
Good.
Then....
> The only issue really is that 99.9% of all testing gets done on x86
> unless you look at specific SoC drivers.
>
> On ARM, for example, there is likely little reason to care about x86
> memory ordering, because there is almost zero driver overlap between
> x86 and ARM.
>
> *Historically*, the reason for following the x86 IO ordering was
> simply that a lot of architectures used the drivers that were
> developed on x86. The alpha and powerpc workstations were *designed*
> with the x86 IO bus (PCI, then PCIe) and to work with the devices that
> came with it.
>
> ARM? PCIe is almost irrelevant. For ARM servers, if they ever take
> off, sure. But 99.99% of ARM is about their own SoC's, and so "x86
> test coverage" is simply not an issue.
>
> How much of an issue is it for Power? Maybe you decide it's not a big deal.
>
> Then all the above is almost irrelevant.
So the overlap may not be that NIL in practice :-) But even then that
doesn't matter as ARM has been happily implementing the same semantic
you describe above for years, as do we powerpc.
This is why, I want (with your agreement) to define clearly and once
and for all, that the Linux semantics of writel are that it is ordered
with previous writes to coherent memory (*)
This is already what ARM and powerpc provide, from what you say, what
x86 provides, I don't see any reason to keep that badly documented and
have drivers randomly growing useless wmb()'s because they don't think
it works on x86 without them !
Once that's sorted, let's tackle the problem of mmiowb vs. spin_unlock
and the problem of writel_relaxed semantics but as separate issues :-)
Also, can I assume the above ordering with writel() equally applies to
readl() or not ?
IE:
dma_buf->foo = 1;
readl(STUPID_DEVICE_DMA_KICK_ON_READ);
Also works on x86 ? (It does on power, maybe not on ARM).
Cheers,
Ben.
(*) From an Linux API perspective, all of this is only valid if the
memory was allocated by dma_alloc_coherent(). Anything obtained by
dma_map_something() might have been bounced bufferred or might require
extra cache flushes on some architectures, and thus needs
dma_sync_for_{cpu,device} calls.
Cheers,
Ben.
next prev parent reply other threads:[~2018-03-28 4:34 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 [this message]
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
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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