This is a note to let you know that I've just added the patch titled
ipmi: kcs: Poll OBF briefly to reduce OBE latency
to the 5.15-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
ipmi-kcs-poll-obf-briefly-to-reduce-obe-latency.patch
and it can be found in the queue-5.15 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 6a6783a60a7dc6c2de783fba08ed10dc4720eff9
Author: Andrew Jeffery <andrew@xxxxxxxx>
Date: Sat Aug 13 00:17:41 2022 +0930
ipmi: kcs: Poll OBF briefly to reduce OBE latency
[ Upstream commit f90bc0f97f2b65af233a37b2e32fc81871a1e3cf ]
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: 28651e6c4237 ("ipmi: kcs_bmc: Allow clients to control KCS IRQ state")
Signed-off-by: Andrew Jeffery <andrew@xxxxxxxx>
Reviewed-by: Joel Stanley <joel@xxxxxxxxx>
Message-Id: <20220812144741.240315-1-andrew@xxxxxxxx>
Signed-off-by: Corey Minyard <cminyard@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/drivers/char/ipmi/kcs_bmc_aspeed.c b/drivers/char/ipmi/kcs_bmc_aspeed.c
index 92a37b33494c..f23c146bb740 100644
--- a/drivers/char/ipmi/kcs_bmc_aspeed.c
+++ b/drivers/char/ipmi/kcs_bmc_aspeed.c
@@ -404,13 +404,31 @@ 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;
/* 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? */
+ 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) {
