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

["Andrew Jeffery" <andrew@aj.id.au>]

On Thu, 6 Oct 2022, at 10:20, Joel Stanley wrote:
> 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...?

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 <andrew@aj.id.au>
>
> Reviewed-by: Joel Stanley <joel@jms.id.au>

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
>>