Re: [PATCH v6 07/10] arm_mpam: prepare mon_sel locking for MPAM-Fb

[Jonathan Cameron]

On Thu, 30 Jul 2026 17:25:36 +0200
Andre Przywara <andre.przywara@xxxxxxx> wrote:

> The MSC MON_SEL register needs to be accessed from hardirq for the overflow
> interrupt, and when taking an IPI to access these registers on platforms
> where MSCs are not accesible from every CPU. This makes an irqsave
> spinlock the obvious lock to protect these registers. On systems with
> MPAM-Fb mailbox MSC access it must be able to sleep, meaning a mutex must
> be used. So MPAM-Fb platforms cannot support an overflow interrupt easily.
> Clearly these two methods can't exist for one MSC at the same time.
> 
> Change the mon_sel locking wrapper function to only use a spinlock when
> the MSC is accessed directly via MMIO. In case of MPAM-Fb, we use a
> mutex, but only if we are in a sleepable context. If that's not the
> case, we return an error. This should not happen, as MPAM-Fb by design
> does not require an MSC access to happen from a specific CPU, so there
> is no need for any IPIs or preemption disabling to satisfy CPU
> constraints. And since overflow interrupts are not supported at the moment
> anyway, we also wouldn't meet the other case.
> Bailing out early is already happening in rare occasions today.
> 
> Signed-off-by: Andre Przywara <andre.przywara@xxxxxxx>
Not sure why I didn't tag this one before.
Reviewed-by: Jonathan Cameron <jonathan.cameron@xxxxxxxxxxxxxxxx>