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

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Date:	Sun, 02 Jan 2011 10:49:32 -0800
From:	David Brown <davidb@...eaurora.org>
To:	Pavel Machek <pavel@....cz>
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, 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.

-- 
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