From: pavel@ucw.cz (Pavel Machek) Date: Mon, 3 Jan 2011 06:50:15 +0100 Subject: [PATCH 1/3] hvc_dcc: Fix bad code generation by marking assembly volatile In-Reply-To: <8yazkrjkuo3.fsf@huya.qualcomm.com> References: <1292649385-28771-1-git-send-email-sboyd@codeaurora.org> <1292875718-7980-1-git-send-email-sboyd@codeaurora.org> <1292875718-7980-2-git-send-email-sboyd@codeaurora.org> <20110102090019.GC32469@atrey.karlin.mff.cuni.cz> <8yazkrjkuo3.fsf@huya.qualcomm.com> Message-ID: <20110103055015.GA19801@elf.ucw.cz> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org 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