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Date:	Sat, 24 Aug 2013 22:35:34 +0200
From:	Richard Weinberger <richard@....at>
To:	Ezequiel Garcia <ezequiel.garcia@...e-electrons.com>
CC:	richard -rw- weinberger <richard.weinberger@...il.com>,
	"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

Am 24.08.2013 21:58, schrieb Ezequiel Garcia:
> 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/ ?

As the kernel is GPL it is the default case to mark new things as GPL symbols.
If your new feature is a core feature which is used by mostly everyone, EXPORT_SYMBOL()
is appropriate.
I.e. having kmalloc() and friends EXPORT_SYMBOL_GPL() would be a bad idea. :)
EXPORT_SYMBOL() seems to be often used in lib/ because most lib/ things are core features.

Thanks,
//richard
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