From: Paul Burton <paul.burton@mips.com>
To: "Maciej W. Rozycki" <macro@linux-mips.org>
Cc: Ralf Baechle <ralf@linux-mips.org>,
"linux-mips@linux-mips.org" <linux-mips@linux-mips.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 0/4] MIPS: Ordering enforcement fixes for MMIO accessors
Date: Thu, 11 Oct 2018 16:40:55 +0000 [thread overview]
Message-ID: <20181011164053.3irkm5dvl7sjzhao@pburton-laptop> (raw)
In-Reply-To: <alpine.LFD.2.21.1810070229190.7757@eddie.linux-mips.org>
Hi Maciej,
On Mon, Oct 08, 2018 at 01:36:55AM +0100, Maciej W. Rozycki wrote:
> This patch series is a follow-up to my earlier consideration about MMIO
> access ordering recorded here: <https://lkml.org/lkml/2014/4/28/201>.
>
> As I have learnt in a recent Alpha/Linux discussion starting here:
> <https://marc.info/?i=alpine.LRH.2.02.1808161556450.13597%20()%20file01%20!%20intranet%20!%20prod%20!%20int%20!%20rdu2%20!%20redhat%20!%20com>
> related to MMIO accessor ordering barriers ports are actually required to
> follow the x86 strongly ordered semantics. As the ordering is not
> specified in the MIPS architecture except for the SYNC instruction we do
> have to put explicit barriers in MMIO accessors as otherwise ordering may
> not be guaranteed.
>
> Fortunately on strongly ordered systems SYNC is expected to be as cheap
> as a NOP, and on weakly ordered ones it is needed anyway. As from
> revision 2.60 of the MIPS architecture specification however we have a
> number of SYNC operations defined, and SYNC 0 has been upgraded from an
> ordering to a completion barrier. We currently don't make use of these
> extra operations and always use SYNC 0 instead, which this means that we
> may be doing too much synchronisation with the barriers we have already
> defined.
>
> This patch series does not make an attempt to optimise for SYNC operation
> use, which belongs to a separate improvement. Instead it focuses on
> fixing MMIO accesses so that drivers can rely on our own API definition.
Agreed, using the lightweight sync types is a whole other can of worms.
I did speak with the architecture team about the description of SYNC
recently (in the context of nanoMIPS documentation if I recall) and hope
the tweaks that were made to the architectural description of it might
help with using them one day soon.
> Following the original consideration specific MMIO barrier operations are
> added. As they have turned out to be required to be implied by MMIO
> accessors there is no immediate need to make them form a generic
> cross-architecture internal Linux API. Therefore I defined them for the
> MIPS architecture only, using the names originally coined by mostly taking
> them from the PowerPC port.
>
> Then I have used them to fix `mmiowb', and then `readX' and `writeX'
> accessors. Finally I have updated the `_relaxed' accessors to avoid
> unnecessary synchronisation WRT DMA.
Thanks - this definitely leaves us in a better place than we were :)
All 4 patches applied to mips-next for 4.20.
Paul
prev parent reply other threads:[~2018-10-11 16:41 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-10-08 0:36 [PATCH 0/4] MIPS: Ordering enforcement fixes for MMIO accessors Maciej W. Rozycki
2018-10-08 0:37 ` [PATCH 1/4] MIPS: Define MMIO ordering barriers Maciej W. Rozycki
2018-10-08 0:37 ` [PATCH 2/4] MIPS: Correct `mmiowb' barrier for `wbflush' platforms Maciej W. Rozycki
2018-10-08 0:37 ` [PATCH 3/4] MIPS: Enforce strong ordering for MMIO accessors Maciej W. Rozycki
2018-10-08 0:37 ` [PATCH 4/4] MIPS: Provide actually relaxed " Maciej W. Rozycki
2018-10-11 16:40 ` Paul Burton [this message]
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20181011164053.3irkm5dvl7sjzhao@pburton-laptop \
--to=paul.burton@mips.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mips@linux-mips.org \
--cc=macro@linux-mips.org \
--cc=ralf@linux-mips.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).