Re: [PATCH v6 4/5] drivers: clocksource: add support for ARM architected timer event stream

[Catalin Marinas <catalin.marinas@arm.com>]

On Thu, Aug 29, 2013 at 03:55:41PM +0100, Sudeep KarkadaNagesha wrote:
> From: Will Deacon <will.deacon@arm.com>
> 
> The ARM architected timer can generate events (used for waking up
> CPUs executing the wfe instruction) at a frequency represented as a
> power-of-2 divisor of the clock rate.
> 
> An event stream might be used:
> - To impose a time-out on a wfe polling loop.
> - To safeguard against any programming error in case an expected event
>   is not generated.
> - To implement wfe-based timeouts for userspace locking implementations.
> 
> This patch computes the event stream frequency aiming for a period
> of 100us between events. It uses ARM/ARM64 specific backends to configure
> and enable the event stream.
> 
> Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Signed-off-by: Will Deacon <will.deacon@arm.com>
> [sudeep: moving ARM/ARM64 changes into separate patches
>          and adding Kconfig option]
> Signed-off-by: Sudeep KarkadaNagesha <sudeep.karkadanagesha@arm.com>
> ---
>  drivers/clocksource/Kconfig          | 14 ++++++++++++++
>  drivers/clocksource/arm_arch_timer.c | 15 +++++++++++++++
>  include/clocksource/arm_arch_timer.h |  2 ++
>  3 files changed, 31 insertions(+)
> 
> diff --git a/drivers/clocksource/Kconfig b/drivers/clocksource/Kconfig
> index b7b9b04..49f1f6e 100644
> --- a/drivers/clocksource/Kconfig
> +++ b/drivers/clocksource/Kconfig
> @@ -74,6 +74,20 @@ config ARM_ARCH_TIMER
>  	bool
>  	select CLKSRC_OF if OF
>  
> +config ARM_ARCH_TIMER_EVTSTREAM
> +	bool "Support for ARM architected timer event stream generation"
> +	default y if ARM_ARCH_TIMER
> +	help
> +	  This option enables support for event stream generation based on
> +	  the ARM architected timer. It is used for waking up CPUs executing
> +	  the wfe instruction at a frequency represented as a power-of-2
> +	  divisor of the clock rate. An event stream might be mainly useful
> +	  for wfe-based timeouts for userspace locking implementations.

I would be a bit more assertive here, something like "the main use of
the event stream is wfe-based timeouts of userspace locking
implementations."

> +	  It might also be useful for imposing timeout on a wfe or even
> +	  safeguarding against any programming errors in case an expected

Maybe "imposting timeout on wfe to safeguard against ...". We already
mentioned wfe-based timeout above.

Otherwise, for the whole series:

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>