Re: [PATCH V2 2/4] drivers: qcom: rpmh-rsc: avoid locking in the interrupt handler

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Quoting Lina Iyer (2019-07-24 07:52:51)
> On Tue, Jul 23 2019 at 14:11 -0600, Stephen Boyd wrote:
> >Quoting Lina Iyer (2019-07-22 14:53:38)
> >> Avoid locking in the interrupt context to improve latency. Since we
> >> don't lock in the interrupt context, it is possible that we now could
> >> race with the DRV_CONTROL register that writes the enable register and
> >> cleared by the interrupt handler. For fire-n-forget requests, the
> >> interrupt may be raised as soon as the TCS is triggered and the IRQ
> >> handler may clear the enable bit before the DRV_CONTROL is read back.
> >>
> >> Use the non-sync variant when enabling the TCS register to avoid reading
> >> back a value that may been cleared because the interrupt handler ran
> >> immediately after triggering the TCS.
> >>
> >> Signed-off-by: Lina Iyer <ilina@xxxxxxxxxxxxxx>
> >> ---
> >
> >I have to read this patch carefully. The commit text isn't convincing me
> >that it is actually safe to make this change. It mostly talks about the
> >performance improvements and how we need to fix __tcs_trigger(), which
> >is good, but I was hoping to be convinced that not grabbing the lock
> >here is safe.
> >
> >How do we ensure that drv->tcs_in_use is cleared before we call
> >tcs_write() and try to look for a free bit? Isn't it possible that we'll
> >get into a situation where the bitmap is all used up but the hardware
> >has just received an interrupt and is going to clear out a bit and then
> >an rpmh write fails with -EBUSY?
> >
> If we have a situation where there are no available free bits, we retry
> and that is part of the function. Since we have only 2 TCSes avaialble
> to write to the hardware and there could be multiple requests coming in,
> it is a very common situation. We try and acquire the drv->lock and if
> there are free TCS available and if available mark them busy and send
> our requests. If there are none available, we keep retrying.

Ok. I wonder if we need some sort of barriers here too, like an
smp_mb__after_atomic()? That way we can make sure that the write to
clear the bit is seen by another CPU that could be spinning forever
waiting for that bit to be cleared? Before this change the spinlock
would be guaranteed to make these barriers for us, but now that doesn't
seem to be the case. I really hope that this whole thing can be changed
to be a mutex though, in which case we can use the bit_wait() API, etc.
to put tasks to sleep while RPMh is processing things.

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