linux-kernel - Re: [PATCH v4 1/4] lib: Introduce atomic MMIO modify

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Message-ID: <20130824195858.GA5957@localhost>
Date:	Sat, 24 Aug 2013 16:58:59 -0300
From:	Ezequiel Garcia <ezequiel.garcia@...e-electrons.com>
To:	richard -rw- weinberger <richard.weinberger@...il.com>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>,
	Gregory Clement <gregory.clement@...e-electrons.com>,
	Lior Amsalem <alior@...vell.com>,
	Baruch Siach <baruch@...s.co.il>,
	Will Deacon <will.deacon@....com>,
	Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>,
	Russell King <linux@....linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>
Subject: Re: [PATCH v4 1/4] lib: Introduce atomic MMIO modify

On Sat, Aug 24, 2013 at 08:27:10PM +0200, richard -rw- weinberger wrote:
> On Sat, Aug 24, 2013 at 5:35 PM, Ezequiel Garcia
> <ezequiel.garcia@...e-electrons.com> wrote:
> > Some platforms have MMIO regions that are shared across orthogonal
> > subsystems. This commit implements a possible solution for the
> > thread-safe access of such regions through a spinlock-protected API.
> >
> > Concurrent access is protected with a single spinlock for the
> > entire MMIO address space. While this protects shared-registers,
> > it also serializes access to unrelated/unshared registers.
> >
> > Signed-off-by: Ezequiel Garcia <ezequiel.garcia@...e-electrons.com>
> > ---
> >  include/linux/io.h |  5 +++++
> >  lib/Makefile       |  2 +-
> >  lib/atomicio.c     | 27 +++++++++++++++++++++++++++
> >  3 files changed, 33 insertions(+), 1 deletion(-)
> >  create mode 100644 lib/atomicio.c
> >
> > diff --git a/include/linux/io.h b/include/linux/io.h
> > index f4f42fa..c331dcb 100644
> > --- a/include/linux/io.h
> > +++ b/include/linux/io.h
> > @@ -101,4 +101,9 @@ static inline void arch_phys_wc_del(int handle)
> >  #define arch_phys_wc_add arch_phys_wc_add
> >  #endif
> >
> > +#ifndef __HAVE_ARCH_ATOMIC_IO_MODIFY
> > +/* Atomic MMIO-wide IO modify */
> > +extern void atomic_io_modify(void __iomem *reg, u32 mask, u32 set);
> > +#endif
> > +
> >  #endif /* _LINUX_IO_H */
> > diff --git a/lib/Makefile b/lib/Makefile
> > index 7baccfd..695d6e2 100644
> > --- a/lib/Makefile
> > +++ b/lib/Makefile
> > @@ -13,7 +13,7 @@ lib-y := ctype.o string.o vsprintf.o cmdline.o \
> >          sha1.o md5.o irq_regs.o reciprocal_div.o argv_split.o \
> >          proportions.o flex_proportions.o prio_heap.o ratelimit.o show_mem.o \
> >          is_single_threaded.o plist.o decompress.o kobject_uevent.o \
> > -        earlycpio.o percpu-refcount.o
> > +        earlycpio.o percpu-refcount.o atomicio.o
> >
> >  obj-$(CONFIG_ARCH_HAS_DEBUG_STRICT_USER_COPY_CHECKS) += usercopy.o
> >  lib-$(CONFIG_MMU) += ioremap.o
> > diff --git a/lib/atomicio.c b/lib/atomicio.c
> > new file mode 100644
> > index 0000000..1750f9d
> > --- /dev/null
> > +++ b/lib/atomicio.c
> > @@ -0,0 +1,27 @@
> > +#include <linux/io.h>
> > +#include <linux/spinlock.h>
> > +
> > +#ifndef __HAVE_ARCH_ATOMIC_IO_MODIFY
> > +/*
> > + * Generic atomic MMIO modify.
> > + *
> > + * Allows thread-safe access to registers shared by unrelated subsystems.
> > + * The access is protected by a single MMIO-wide lock.
> > + *
> > + * Optimized variants can be implemented on a per-architecture basis.
> > + */
> > +static DEFINE_RAW_SPINLOCK(__io_lock);
> > +void atomic_io_modify(void __iomem *reg, u32 mask, u32 set)
> > +{
> > +       unsigned long flags;
> > +       u32 value;
> > +
> > +       raw_spin_lock_irqsave(&__io_lock, flags);
> > +       value = readl(reg) & ~mask;
> > +       value |= (set & mask);
> > +       writel(value, reg);
> > +       raw_spin_unlock_irqrestore(&__io_lock, flags);
> > +
> > +}
> > +EXPORT_SYMBOL(atomic_io_modify);
> 
> Why not the default case EXPORT_SYMBOL_GPL()?
> 

Because I copy-pasted the export from some other lib/.. :-)

Mind explaining me the difference, why you say _GPL it's the default,
and why EXPORT_SYMBOL is more frequently used in lib/ ?

$ git grep EXPORT_SYMBOL lib/ | wc -l
598
$ git grep EXPORT_SYMBOL_GPL lib/ | wc -l
117

Thanks!
-- 
Ezequiel García, Free Electrons
Embedded Linux, Kernel and Android Engineering
http://free-electrons.com
--
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