Re: [PATCH] ipmi: kcs: Poll OBF briefly to reduce OBE latency

[Corey Minyard]

On Thu, Oct 06, 2022 at 01:36:51PM +1030, Andrew Jeffery wrote:
> 
> 
> On Thu, 6 Oct 2022, at 10:20, Joel Stanley wrote:
> > On Fri, 12 Aug 2022 at 14:48, Andrew Jeffery <andrew@xxxxxxxx> wrote:
> >>
> >> The ASPEED KCS devices don't provide a BMC-side interrupt for the host
> >> reading the output data register (ODR). The act of the host reading ODR
> >> clears the output buffer full (OBF) flag in the status register (STR),
> >> informing the BMC it can transmit a subsequent byte.
> >>
> >> On the BMC side the KCS client must enable the OBE event *and* perform a
> >> subsequent read of STR anyway to avoid races - the polling provides a
> >> window for the host to read ODR if data was freshly written while
> >> minimising BMC-side latency.
> >>
> >
> > Fixes...?
> 
> Is it a fix though? It's definitely an *improvement* in behaviour, but 
> the existing behaviour also wasn't *incorrect*, just kinda unfortunate 
> under certain timings? Dunno. I'm probably splitting hairs.
> 
> In any case, if we do want a fixes line:
> 
> Fixes: 28651e6c4237 ("ipmi: kcs_bmc: Allow clients to control KCS IRQ state")

I added the Fixes and Joel's review.

Thanks,

-corey

> 
> >
> >> Signed-off-by: Andrew Jeffery <andrew@xxxxxxxx>
> >
> > Reviewed-by: Joel Stanley <joel@xxxxxxxxx>
> 
> Thanks!
> 
> >
> >> ---
> >>  drivers/char/ipmi/kcs_bmc_aspeed.c | 24 +++++++++++++++++++++---
> >>  1 file changed, 21 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/drivers/char/ipmi/kcs_bmc_aspeed.c b/drivers/char/ipmi/kcs_bmc_aspeed.c
> >> index cdc88cde1e9a..417e5a3ccfae 100644
> >> --- a/drivers/char/ipmi/kcs_bmc_aspeed.c
> >> +++ b/drivers/char/ipmi/kcs_bmc_aspeed.c
> >> @@ -399,13 +399,31 @@ static void aspeed_kcs_check_obe(struct timer_list *timer)
> >>  static void aspeed_kcs_irq_mask_update(struct kcs_bmc_device *kcs_bmc, u8 mask, u8 state)
> >>  {
> >>         struct aspeed_kcs_bmc *priv = to_aspeed_kcs_bmc(kcs_bmc);
> >> +       int rc;
> >> +       u8 str;
> >
> > str is status, it would be good to spell that out in full.
> 
> I guess if it trips enough people up we can rename it later.
> 
> >
> >>
> >>         /* We don't have an OBE IRQ, emulate it */
> >>         if (mask & KCS_BMC_EVENT_TYPE_OBE) {
> >> -               if (KCS_BMC_EVENT_TYPE_OBE & state)
> >> -                       mod_timer(&priv->obe.timer, jiffies + OBE_POLL_PERIOD);
> >> -               else
> >> +               if (KCS_BMC_EVENT_TYPE_OBE & state) {
> >> +                       /*
> >> +                        * Given we don't have an OBE IRQ, delay by polling briefly to see if we can
> >> +                        * observe such an event before returning to the caller. This is not
> >> +                        * incorrect because OBF may have already become clear before enabling the
> >> +                        * IRQ if we had one, under which circumstance no event will be propagated
> >> +                        * anyway.
> >> +                        *
> >> +                        * The onus is on the client to perform a race-free check that it hasn't
> >> +                        * missed the event.
> >> +                        */
> >> +                       rc = read_poll_timeout_atomic(aspeed_kcs_inb, str,
> >> +                                                     !(str & KCS_BMC_STR_OBF), 1, 100, false,
> >> +                                                     &priv->kcs_bmc, priv->kcs_bmc.ioreg.str);
> >> +                       /* Time for the slow path? */
> >
> > The mod_timer is the slow path? The question mark threw me.
> 
> Yeah, mod_timer() is the slow path; read_poll_timeout_atomic() is the 
> fast path and we've exhausted the time we're willing to wait there if 
> we get -ETIMEDOUT.
> 
> The comment was intended as a description for the question posed by the 
> condition. It made sense in my head but maybe it's confusing more than 
> it is enlightening?
> 
> Andrew
> 
> >
> >> +                       if (rc == -ETIMEDOUT)
> >> +                               mod_timer(&priv->obe.timer, jiffies + OBE_POLL_PERIOD);
> >> +               } else {
> >>                         del_timer(&priv->obe.timer);
> >> +               }
> >>         }
> >>
> >>         if (mask & KCS_BMC_EVENT_TYPE_IBF) {
> >> --
> >> 2.34.1
> >>