From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Jeffery Date: Thu, 06 Oct 2022 13:36:51 +1030 Subject: [PATCH] ipmi: kcs: Poll OBF briefly to reduce OBE latency In-Reply-To: References: <20220812144741.240315-1-andrew@aj.id.au> Message-ID: List-Id: To: linux-aspeed@lists.ozlabs.org MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit On Thu, 6 Oct 2022, at 10:20, Joel Stanley wrote: > On Fri, 12 Aug 2022 at 14:48, Andrew Jeffery 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") > >> Signed-off-by: Andrew Jeffery > > Reviewed-by: Joel Stanley 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 >>