From: Pavel Machek <pavel@ucw.cz>
To: David Brown <davidb@codeaurora.org>
Cc: Nicolas Pitre <nico@fluxnic.net>,
Stephen Boyd <sboyd@codeaurora.org>,
Greg Kroah-Hartman <gregkh@suse.de>,
linux-kernel@vger.kernel.org, linux-arm-msm@vger.kernel.org,
linux-arm-kernel@lists.infradead.org,
Tony Lindgren <tony@atomide.com>, Arnd Bergmann <arnd@arndb.de>,
Daniel Walker <dwalker@codeaurora.org>
Subject: Re: [PATCH 1/3] hvc_dcc: Fix bad code generation by marking assembly volatile
Date: Mon, 3 Jan 2011 06:50:15 +0100 [thread overview]
Message-ID: <20110103055015.GA19801@elf.ucw.cz> (raw)
In-Reply-To: <8yazkrjkuo3.fsf@huya.qualcomm.com>
On Sun 2011-01-02 10:49:32, David Brown wrote:
> On Sun, Jan 02 2011, Pavel Machek wrote:
>
> >> > diff --git a/drivers/char/hvc_dcc.c b/drivers/char/hvc_dcc.c
> >> > index 6470f63..155ec10 100644
> >> > --- a/drivers/char/hvc_dcc.c
> >> > +++ b/drivers/char/hvc_dcc.c
> >> > @@ -33,8 +33,7 @@
> >> > static inline u32 __dcc_getstatus(void)
> >> > {
> >> > u32 __ret;
> >> > -
> >> > - asm("mrc p14, 0, %0, c0, c1, 0 @ read comms ctrl reg"
> >> > + asm volatile("mrc p14, 0, %0, c0, c1, 0 @ read comms ctrl reg"
> >> > : "=r" (__ret) : : "cc");
> >> >
> >> > return __ret;
> >
> > Is volatile needed here? If __dcc_getstatus() return value is
> > discarded, we want assembly discarded, right?
>
> That's not really the issue being fixed. Without the volatile, the
> compiler is free to cache and reuse a previously loaded status value.
> It is important that the status be read each time.
>
> I don't think there is a way of indicating that assembly needs to happen
> for each use, but that it is OK to discard if the value isn't used.
> 'volatile' is a bit overloaded.
Ok, thanks for explanation.
Pavel
--
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
WARNING: multiple messages have this Message-ID (diff)
From: pavel@ucw.cz (Pavel Machek)
To: linux-arm-kernel@lists.infradead.org
Subject: [PATCH 1/3] hvc_dcc: Fix bad code generation by marking assembly volatile
Date: Mon, 3 Jan 2011 06:50:15 +0100 [thread overview]
Message-ID: <20110103055015.GA19801@elf.ucw.cz> (raw)
In-Reply-To: <8yazkrjkuo3.fsf@huya.qualcomm.com>
On Sun 2011-01-02 10:49:32, David Brown wrote:
> On Sun, Jan 02 2011, Pavel Machek wrote:
>
> >> > diff --git a/drivers/char/hvc_dcc.c b/drivers/char/hvc_dcc.c
> >> > index 6470f63..155ec10 100644
> >> > --- a/drivers/char/hvc_dcc.c
> >> > +++ b/drivers/char/hvc_dcc.c
> >> > @@ -33,8 +33,7 @@
> >> > static inline u32 __dcc_getstatus(void)
> >> > {
> >> > u32 __ret;
> >> > -
> >> > - asm("mrc p14, 0, %0, c0, c1, 0 @ read comms ctrl reg"
> >> > + asm volatile("mrc p14, 0, %0, c0, c1, 0 @ read comms ctrl reg"
> >> > : "=r" (__ret) : : "cc");
> >> >
> >> > return __ret;
> >
> > Is volatile needed here? If __dcc_getstatus() return value is
> > discarded, we want assembly discarded, right?
>
> That's not really the issue being fixed. Without the volatile, the
> compiler is free to cache and reuse a previously loaded status value.
> It is important that the status be read each time.
>
> I don't think there is a way of indicating that assembly needs to happen
> for each use, but that it is OK to discard if the value isn't used.
> 'volatile' is a bit overloaded.
Ok, thanks for explanation.
Pavel
--
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
next prev parent reply other threads:[~2011-01-03 5:50 UTC|newest]
Thread overview: 79+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-11-30 19:25 [PATCH] drivers: char: hvc: add arm JTAG DCC console support Daniel Walker
2010-11-30 19:25 ` Daniel Walker
2010-11-30 19:57 ` Nicolas Pitre
2010-11-30 19:57 ` Nicolas Pitre
2010-11-30 21:17 ` Arnd Bergmann
2010-11-30 21:17 ` Arnd Bergmann
2010-12-01 5:30 ` Stephen Boyd
2010-12-01 5:30 ` Stephen Boyd
2010-12-01 18:54 ` Daniel Walker
2010-12-01 18:54 ` Daniel Walker
2010-12-01 19:28 ` Greg KH
2010-12-01 19:28 ` Greg KH
2010-12-18 5:16 ` [PATCH] hvc_dcc: Simplify assembly for v6 and v7 ARM Stephen Boyd
2010-12-18 5:16 ` Stephen Boyd
2010-12-20 17:51 ` Daniel Walker
2010-12-20 17:51 ` Daniel Walker
2010-12-20 18:39 ` Stephen Boyd
2010-12-20 18:39 ` Stephen Boyd
2010-12-20 18:46 ` Nicolas Pitre
2010-12-20 18:46 ` Nicolas Pitre
2010-12-20 20:08 ` [PATCH 0/3] hvc_dcc cleanups and fixes Stephen Boyd
2010-12-20 20:08 ` Stephen Boyd
2010-12-20 20:08 ` [PATCH 1/3] hvc_dcc: Fix bad code generation by marking assembly volatile Stephen Boyd
2010-12-20 20:08 ` Stephen Boyd
2010-12-20 21:39 ` Nicolas Pitre
2010-12-20 21:39 ` Nicolas Pitre
2011-01-02 9:00 ` Pavel Machek
2011-01-02 9:00 ` Pavel Machek
2011-01-02 18:49 ` David Brown
2011-01-02 18:49 ` David Brown
2011-01-03 5:50 ` Pavel Machek [this message]
2011-01-03 5:50 ` Pavel Machek
2011-01-04 18:49 ` Tony Lindgren
2011-01-04 18:49 ` Tony Lindgren
2010-12-20 21:49 ` Arnaud Lacombe
2010-12-20 21:49 ` Arnaud Lacombe
2010-12-20 21:52 ` Stephen Boyd
2010-12-20 21:52 ` Stephen Boyd
2010-12-20 22:10 ` Nicolas Pitre
2010-12-20 22:10 ` Nicolas Pitre
2010-12-20 20:08 ` [PATCH 2/3] hvc_dcc: Simplify put_chars()/get_chars() loops Stephen Boyd
2010-12-20 20:08 ` Stephen Boyd
2010-12-20 20:08 ` [PATCH 3/3] hvc_dcc: Simplify assembly for v6 and v7 ARM Stephen Boyd
2010-12-20 20:08 ` Stephen Boyd
2010-12-20 21:44 ` Nicolas Pitre
2010-12-20 21:44 ` Nicolas Pitre
2011-01-04 18:52 ` Tony Lindgren
2011-01-04 18:52 ` Tony Lindgren
2011-01-06 1:49 ` [PATCH 0/3] hvc_dcc cleanups and fixes Stephen Boyd
2011-01-06 1:49 ` Stephen Boyd
2011-01-06 3:20 ` Greg KH
2011-01-06 3:20 ` Greg KH
2011-02-03 22:17 ` Greg KH
2011-02-03 22:17 ` Greg KH
2011-02-03 22:17 ` Greg KH
2011-02-03 23:19 ` Stephen Boyd
2011-02-03 23:19 ` Stephen Boyd
2011-02-03 23:30 ` Greg KH
2011-02-03 23:30 ` Greg KH
2011-02-03 23:48 ` [PATCHv2 " Stephen Boyd
2011-02-03 23:48 ` Stephen Boyd
2011-02-03 23:48 ` [PATCHv2 1/3] hvc_dcc: Fix bad code generation by marking assembly volatile Stephen Boyd
2011-02-03 23:48 ` Stephen Boyd
2011-02-03 23:48 ` [PATCHv2 2/3] hvc_dcc: Simplify put_chars()/get_chars() loops Stephen Boyd
2011-02-03 23:48 ` Stephen Boyd
2011-02-03 23:48 ` [PATCHv2 3/3] hvc_dcc: Simplify assembly for v6 and v7 ARM Stephen Boyd
2011-02-03 23:48 ` Stephen Boyd
2010-12-01 20:20 ` [PATCH] drivers: char: hvc: add arm JTAG DCC console support Stephen Boyd
2010-12-01 20:20 ` Stephen Boyd
2010-12-07 19:10 ` Stephen Boyd
2010-12-07 19:10 ` Stephen Boyd
2011-01-14 19:19 ` Tony Lindgren
2011-01-14 19:19 ` Tony Lindgren
2011-01-14 23:49 ` Stephen Boyd
2011-01-14 23:49 ` Stephen Boyd
2011-04-07 18:39 ` RONETIX - Asen Dimov
2011-04-07 18:39 ` RONETIX - Asen Dimov
2011-04-07 18:57 ` Mike Frysinger
2011-04-07 18:57 ` Mike Frysinger
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