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ipmi: kcs: Poll OBF briefly to reduce OBE latency 

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: 28651e6c42 ("ipmi: kcs_bmc: Allow clients to control KCS IRQ state")
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Message-Id: <20220812144741.240315-1-andrew@aj.id.au>
Signed-off-by: Corey Minyard <cminyard@mvista.com>
This commit is contained in:
Andrew Jeffery 2022-08-13 00:17:41 +09:30 committed by Corey Minyard
parent c25c782d34
commit f90bc0f97f
1 changed files with 21 additions and 3 deletions
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24
drivers/char/ipmi/kcs_bmc_aspeed.c
View File

@ -406,13 +406,31 @@ 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) {