From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <benh@kernel.crashing.org>
Received: from smtp1.linuxfoundation.org (smtp1.linux-foundation.org
	[172.17.192.35])
	by mail.linuxfoundation.org (Postfix) with ESMTPS id C5D9CB5A
	for <ksummit-discuss@lists.linuxfoundation.org>;
	Sun,  5 Jul 2015 19:56:51 +0000 (UTC)
Received: from gate.crashing.org (gate.crashing.org [63.228.1.57])
	by smtp1.linuxfoundation.org (Postfix) with ESMTPS id A51F2168
	for <ksummit-discuss@lists.linuxfoundation.org>;
	Sun,  5 Jul 2015 19:56:49 +0000 (UTC)
Message-ID: <1436126184.3948.55.camel@kernel.crashing.org>
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Andy Lutomirski <luto@amacapital.net>
Date: Mon, 06 Jul 2015 05:56:24 +1000
In-Reply-To: <CALCETrV82gJKhYbvdrqt3SHE6=42=QBKD8BcQPSukk2nzi1+EA@mail.gmail.com>
References: <1435997837.3948.21.camel@kernel.crashing.org>
	<CAPcyv4hsJDJwv-yEJiiy2ptn7QYbCA1+3s9gTQuhz_C9UKGGsg@mail.gmail.com>
	<1436065368.3948.48.camel@kernel.crashing.org>
	<CALCETrV82gJKhYbvdrqt3SHE6=42=QBKD8BcQPSukk2nzi1+EA@mail.gmail.com>
Content-Type: text/plain; charset="UTF-8"
Mime-Version: 1.0
Content-Transfer-Encoding: 7bit
Cc: "ksummit-discuss@lists.linuxfoundation.org"
	<ksummit-discuss@lists.linuxfoundation.org>
Subject: Re: [Ksummit-discuss] [TECH TOPIC] Semantics of MMIO mapping
 attributes accross archs
List-Id: <ksummit-discuss.lists.linuxfoundation.org>
List-Unsubscribe: <https://lists.linuxfoundation.org/mailman/options/ksummit-discuss>,
	<mailto:ksummit-discuss-request@lists.linuxfoundation.org?subject=unsubscribe>
List-Archive: <http://lists.linuxfoundation.org/pipermail/ksummit-discuss/>
List-Post: <mailto:ksummit-discuss@lists.linuxfoundation.org>
List-Help: <mailto:ksummit-discuss-request@lists.linuxfoundation.org?subject=help>
List-Subscribe: <https://lists.linuxfoundation.org/mailman/listinfo/ksummit-discuss>,
	<mailto:ksummit-discuss-request@lists.linuxfoundation.org?subject=subscribe>

On Sun, 2015-07-05 at 11:55 -0700, Andy Lutomirski wrote:
> 
> At some point, it would also be nice if the various macros has
> well-defined semantics.  For example, x86 has:
> 
> #define pgprot_noncached(prot)                                          \
>         ((boot_cpu_data.x86 > 3)                                        \
>          ? (__pgprot(pgprot_val(prot) |                                 \
>                      cachemode2protval(_PAGE_CACHE_MODE_UC_MINUS)))     \
>          : (prot))
> 
> Putting aside the pointless boot_cpu_data check (surely the recent PAT
> rework completely obsoletes it), what is
> pgprot_noncached(pgprot_writecombine(x)) supposed to do?  Currently it
> results in garbage.  Should it have well-defined behavior instead?

Can it ? On powerpc it will just mean pgprot_noncached for example,
those macros manipulate the same bits and it's not a bitmask, it's
either unached or uncached with write combining.

> I suspect the other arches all have their own unique glitches here.

Correct. I'm still trying to get feedback on ARM for example.

I don't think we can (or should try) to have completely identical semantics
for everything, but we should try to find the common set that are guaranteed
and, possibly, do a best effort for archs to individually document the
remaining.

Ben.