Re: [PATCH v2 2/3] mwifiex: pcie: don't loop/retry interrupt status checks

[Brian Norris]

On Sun, Jan 15, 2017 at 04:54:52PM -0800, Dmitry Torokhov wrote:
> On Fri, Jan 13, 2017 at 03:35:37PM -0800, Brian Norris wrote:
> > The following sequence occurs when using IEEE power-save on 8997:
> > (a) driver sees SLEEP event
> > (b) driver issues SLEEP CONFIRM
> > (c) driver recevies CMD interrupt; within the interrupt processing loop,
> >     we do (d) and (e):
> > (d) wait for FW sleep cookie (and often time out; it takes a while), FW
> >     is putting card into low power mode
> > (e) re-check PCIE_HOST_INT_STATUS register; quit loop with 0 value
> > 
> > But at (e), no one actually signaled an interrupt (i.e., we didn't check
> > adapter->int_status). And what's more, because the card is going to
> > sleep, this register read appears to take a very long time in some cases
> > -- 3 milliseconds in my case!
> > 
> > Now, I propose that (e) is completely unnecessary. If there were any
> > additional interrupts signaled after the start of this loop, then the
> > interrupt handler would have set adapter->int_status to non-zero and
> > queued more work for the main loop -- and we'd catch it on the next
> > iteration of the main loop.
> > 
> > So this patch drops all the looping/re-reading of PCIE_HOST_INT_STATUS,
> > which avoids the problematic (and slow) register read in step (e).
> > 
> > Incidentally, this is a very similar issue to the one fixed in commit
> > ec815dd2a5f1 ("mwifiex: prevent register accesses after host is
> > sleeping"), except that the register read is just very slow instead of
> > fatal in this case.
> > 
> > Tested on 8997 in both MSI and (though not technically supported at the
> > moment) MSI-X mode.
> 
> Well, that kills interrupt mitigation and with PCIE that might be
> somewhat important (SDIO is too slow to be important I think) and might
> cost you throughput.

Hmm, well I don't see us disabling interrupts in here, at least for MSI
mode, so it doesn't actually look like a mitigation mechanism. More like
a redundancy. But I'm not an expert on MSI, and definitely not on
network performance.

Also, FWIW, I did some fairly non-scientific tests of this on my
systems, and I didn't see much difference. I can run better tests, and
even collect data on how often we loop here vs. see new interrupts.

> OTOH maybe Marvell should convert PICE to NAPI to make this more
> obvious and probably more correct.

>From my brief reading, that sounds like a better way to make this
configurable.

So I'm not sure which way you'd suggest then; take a patch like this,
which makes the driver more clear and less buggy? Or write some
different patch that isolates just the power-save related condition, so
we break out of this look [1]?

I'm also interested in any opinions from the Marvell side -- potentially
testing results, rationale behind this code structure, or even a better
alternative patch.

Brian

[1] i.e., along the lines of commit ec815dd2a5f1 ("mwifiex: prevent
register accesses after host is sleeping").