linux-kernel - Re: [PATCH 1/3] hvc_dcc: Fix bad code generation by marking assembly volatile

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Message-ID: <20110103055015.GA19801@elf.ucw.cz>
Date:	Mon, 3 Jan 2011 06:50:15 +0100
From:	Pavel Machek <pavel@....cz>
To:	David Brown <davidb@...eaurora.org>
Cc:	Nicolas Pitre <nico@...xnic.net>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	Tony Lindgren <tony@...mide.com>,
	Arnd Bergmann <arnd@...db.de>,
	Daniel Walker <dwalker@...eaurora.org>
Subject: Re: [PATCH 1/3] hvc_dcc: Fix bad code generation by marking
 assembly volatile

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
-- 
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