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

From: Joel Stanley
Date: Wed Oct 05 2022 - 19:50:42 EST


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...?

> Signed-off-by: Andrew Jeffery <andrew@xxxxxxxx>

Reviewed-by: Joel Stanley <joel@xxxxxxxxx>

> ---
> 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.

>
> /* 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.

> + 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
>