Re: [PATCH 1/3] ARM: Introduce atomic MMIO clear/set
From: Ezequiel Garcia
Date: Sat Aug 10 2013 - 10:02:47 EST
On Sat, Aug 10, 2013 at 04:49:28PM +0400, Alexander Shiyan wrote:
> > Some SoC 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
> > with clear-set semantics.
> >
> > 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@xxxxxxxxxxxxxxxxxx>
> > ---
> > arch/arm/include/asm/io.h | 5 +++++
> > arch/arm/kernel/io.c | 24 ++++++++++++++++++++++++
> > 2 files changed, 29 insertions(+)
> >
> > diff --git a/arch/arm/include/asm/io.h b/arch/arm/include/asm/io.h
> > index d070741..c84658d 100644
> > --- a/arch/arm/include/asm/io.h
> > +++ b/arch/arm/include/asm/io.h
> > @@ -36,6 +36,11 @@
> > #define isa_bus_to_virt phys_to_virt
> >
> > /*
> > + * Atomic MMIO-wide IO clear/set
> > + */
> > +extern void atomic_io_clear_set(void __iomem *reg, u32 clear, u32 set);
> > +
> > +/*
> > * Generic IO read/write. These perform native-endian accesses. Note
> > * that some architectures will want to re-define __raw_{read,write}w.
> > */
> > diff --git a/arch/arm/kernel/io.c b/arch/arm/kernel/io.c
> > index dcd5b4d..3ab8201 100644
> > --- a/arch/arm/kernel/io.c
> > +++ b/arch/arm/kernel/io.c
> > @@ -1,6 +1,30 @@
> > #include <linux/export.h>
> > #include <linux/types.h>
> > #include <linux/io.h>
> > +#include <linux/spinlock.h>
> > +
> > +static DEFINE_SPINLOCK(__io_lock);
> > +
> > +/*
> > + * Some platforms have MMIO regions that are shared across orthogonal
> > + * subsystems. This API implements thread-safe access to such regions
> > + * through a spinlock-protected API with clear-set semantics.
> > + *
> > + * 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.
> > + *
> > + * Using this API on frequently accessed registers in performance-critical
> > + * paths is not recommended, as the spinlock used by this API would become
> > + * highly contended.
> > + */
> > +void atomic_io_clear_set(void __iomem *reg, u32 clear, u32 set)
> > +{
> > + spin_lock(&__io_lock);
> > + writel((readl(reg) & ~clear) | set, reg);
> > + spin_unlock(&__io_lock);
> > +}
> > +EXPORT_SYMBOL(atomic_io_clear_set);
>
> So, one lock is used to all possible registers?
> Seems a regmap-mmio can be used for such access.
>
Thanks for the hint! Quite frankly, I wasn't familiar with regmap-mmio.
However, after reading some code, I fail to see how that helps in this case.
Note that we need to access the *same* MMIO address from completely
different (and unrelated) drivers, such as watchdog and clocksource.
So I wonder who would "own" the regmap descriptor, and how does the other
one gets aware of that descriptor?
In addition given we can use orion_wdt (originally meant for mach-kirkwood)
to support mvebu SoC watchdog, we need to sort this out in a completely
multiplatform capable way.
Ideas?
--
Ezequiel GarcÃa, Free Electrons
Embedded Linux, Kernel and Android Engineering
http://free-electrons.com
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