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

[Lina Iyer]

On Wed, Jul 24 2019 at 13:38 -0600, Stephen Boyd wrote:
> 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.
We have drivers that want to send requests in atomic contexts and
therefore mutex locks would not work.

--Lina