[PATCH v2 01/15] ARM: Introduce atomic MMIO modify

On Tue, Jan 21, 2014 at 10:45:21AM +0100, Arnd Bergmann wrote:

On Tuesday 21 January 2014 06:12:27 Ezequiel Garcia wrote:

quoted

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.

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.

We add relaxed and non-relaxed variants, by using writel_relaxed and writel,
respectively. The rationale for this is that some users may not require
register write completion but only thread-safe access to a register.

Signed-off-by: Ezequiel Garcia <redacted>

You add the new atomic mmio interfaces in an ARM global header file,
but at the same time make them ARM-only. I'm not opposed to having
interfaces like that, but I'm not convinced they are actually needed
for this case and if we go there, it needs to be done more carefully
and should be available for all architectures so that portable drivers
can use them.

Yes, true. We've discussed about this and even post an arch-generic API:

http://lwn.net/Articles/564709/

In the end it was decided to keep it ARM-specific for now:

http://www.spinics.net/lists/arm-kernel/msg271773.html

Just for reference, the last posted patchset for the atomic I/O API
is this:

http://www.spinics.net/lists/arm-kernel/msg292775.html

It also seems to duplicate functionality that is already present in
regmap-mmio.

Yes, this is true. We tried to use regmap-mmio/syscon but we need this
to initialize the clocksource which is too early for that to work:

https://www.mail-archive.com/linux-kernel at vger.kernel.org/msg484535.html

-- 
Ezequiel Garc?a, Free Electrons
Embedded Linux, Kernel and Android Engineering
http://free-electrons.com