From: Corey Minyard <minyard@acm.org>
To: Andrew Jeffery <andrew@aj.id.au>
Cc: Joel Stanley <joel@jms.id.au>,
openipmi-developer@lists.sourceforge.net,
linux-arm-kernel@lists.infradead.org,
linux-aspeed@lists.ozlabs.org, linux-kernel@vger.kernel.org,
openbmc@lists.ozlabs.org
Subject: Re: [PATCH] ipmi: kcs: Poll OBF briefly to reduce OBE latency
Date: Thu, 6 Oct 2022 09:09:57 -0500 [thread overview]
Message-ID: <Yz7htdRvb+IhU060@dell1.minyard.net> (raw)
In-Reply-To: <f6a85e5e-a3b1-4e5c-9db6-1222dcabd780@app.fastmail.com>
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@aj.id.au> 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@aj.id.au>
> >
> > Reviewed-by: Joel Stanley <joel@jms.id.au>
>
> 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
> >>
prev parent reply other threads:[~2022-10-06 14:18 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-08-12 14:47 [PATCH] ipmi: kcs: Poll OBF briefly to reduce OBE latency Andrew Jeffery
2022-10-05 23:12 ` Andrew Jeffery
2022-10-05 23:54 ` Corey Minyard
2022-10-06 3:08 ` Andrew Jeffery
2022-10-05 23:50 ` Joel Stanley
2022-10-06 3:06 ` Andrew Jeffery
2022-10-06 14:09 ` Corey Minyard [this message]
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