From: Joel Stanley <joel@jms.id.au>
To: Andrew Jeffery <andrew@aj.id.au>
Cc: openipmi-developer@lists.sourceforge.net, minyard@acm.org,
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: Wed, 5 Oct 2022 23:50:23 +0000 [thread overview]
Message-ID: <CACPK8XdnFPXZuzAaSMOUnV6ng9y2_ZMcAff12q8Zxvr2HJZy-w@mail.gmail.com> (raw)
In-Reply-To: <20220812144741.240315-1-andrew@aj.id.au>
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...?
> Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Reviewed-by: Joel Stanley <joel@jms.id.au>
> ---
> 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
>
next prev parent reply other threads:[~2022-10-05 23:50 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 [this message]
2022-10-06 3:06 ` Andrew Jeffery
2022-10-06 14:09 ` Corey Minyard
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