RFC: Dynamic hwcaps

[Ben Dooks]

On 03/12/10 16:28, Dave Martin wrote:
> Hi all,
> 
> I'd be interested in people's views on the following idea-- feel free
> to ignore if it doesn't interest you.
> 
> 
> For power-management purposes, it's useful to be able to turn off
> functional blocks on the SoC.
> 
> For on-SoC peripherals, this can be managed through the driver
> framework in the kernel, but for functional blocks of the CPU itself
> which are used by instruction set extensions, such as NEON or other
> media accelerators, it would be interesting if processes could adapt
> to these units appearing and disappearing at runtime.  This would mean
> that user processes would need to select dynamically between different
> implementations of accelerated functionality at runtime.
> 
> This allows for more active power management of such functional
> blocks: if the CPU is not fully loaded, you can turn them off -- the
> kernel can spot when there is significant idle time and do this.  If
> the CPU becomes fully loaded, applications which have soft-realtime
> constraints can notice this and switch to their accelerated code
> (which will cause the kernel to switch the functional unit(s) on).
> Or, the kernel can react to increasing CPU load by speculatively turn
> it on instead.  This is analogous to the behaviour of other power
> governors in the system.  Non-aware applications will still work
> seamlessly -- these may simply run accelerated code if the hardware
> supports it, causing the kernel to turn the affected functional
> block(s) on.
> 
> In order for this to work, some dynamic status information would need
> to be visible to each user process, and polled each time a function
> with a dynamically switchable choice of implementations gets called.
> You probably don't need to worry about race conditions either-- if the
> process accidentally tries to use a turned-off feature, you will take
> a fault which gives the kernel the chance to turn the feature back on.

Could you do what the original FP did, and start with units off and use
the first use of $unit in the process to turn it on? Do things like NEON
support this?