Re: ACPI: EC: Clear GPE on interrupt handling only

["Compostella, Jeremy" <jeremy.compostella@intel.com>]

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"Rafael J. Wysocki" <rafael@kernel.org> writes:

> On Mon, Jun 5, 2023 at 6:14 PM Rafael J. Wysocki <rafael@kernel.org> wrote:
>>
>> On Tue, May 16, 2023 at 2:02 AM Compostella, Jeremy
>> <jeremy.compostella@intel.com> wrote:
>> >
>> > On multiple devices I work on, we noticed that
>> > /sys/firmware/acpi/interrupts/sci_not is non-zero and keeps increasing
>> > over time.
>> >
>> > It turns out that there is a race condition between servicing a GPE
>> > interrupt and handling task driven transactions.
>> >
>> > If a GPE interrupt is received at the same time ec_poll() is running,
>> > the advance_transaction() clears the GPE flag and the interrupt is not
>> > serviced as acpi_ev_detect_gpe() relies on the GPE flag to call the
>> > handler. As a result, `sci_not' is increased.
>>
>> And if I'm not mistaken, it is not necessary to run the entire
>> interrupt handler in that case, because the currently running
>> advance_transaction() will take care of the pending event anyway.
>>
>> I agree that it is confusing to increase sci_not in that case, but I'm
>> not sure if running the entire advance_transaction() for the same
>> transaction twice in a row, once from ec_poll() and once from the
>> interrupt handler is entirely correct.
>
> However, if the interrupt handler wins the race, advance_transaction()
> will run for the same transaction twice in a row anyway, so this
> change will only make it happen more often.
>
> So no objections, but I would move the GPE clearing piece directly
> into acpi_ec_handle_interrupt(), because it will only be needed there
> and it doesn't depend on anything else in advance_transaction().

I took into account your suggestion (cf. patch in attachment).


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From 42fa736fcd5d6a2e17c550f493a12e8df2e7cd72 Mon Sep 17 00:00:00 2001
From: Jeremy Compostella <jeremy.compostella@intel.com>
Date: Mon, 15 May 2023 16:49:19 -0700
Subject: [PATCH] ACPI: EC: Clear GPE on interrupt handling only

On multiple devices I work on, we noticed that
/sys/firmware/acpi/interrupts/sci_not is non-zero and keeps increasing
over time.

It turns out that there is a race condition between servicing a GPE
interrupt and handling task driven transactions.

If a GPE interrupt is received at the same time ec_poll() is running,
the advance_transaction() clears the GPE flag and the interrupt is not
serviced as acpi_ev_detect_gpe() relies on the GPE flag to call the
handler. As a result, `sci_not' is increased.

Signed-off-by: Jeremy Compostella <jeremy.compostella@intel.com>
---
 drivers/acpi/ec.c | 31 ++++++++++++++++---------------
 1 file changed, 16 insertions(+), 15 deletions(-)

diff --git a/drivers/acpi/ec.c b/drivers/acpi/ec.c
index 928899ab9502..8569f55e55b6 100644
--- a/drivers/acpi/ec.c
+++ b/drivers/acpi/ec.c
@@ -662,21 +662,6 @@ static void advance_transaction(struct acpi_ec *ec, bool interrupt)
 
 	ec_dbg_stm("%s (%d)", interrupt ? "IRQ" : "TASK", smp_processor_id());
 
-	/*
-	 * Clear GPE_STS upfront to allow subsequent hardware GPE_STS 0->1
-	 * changes to always trigger a GPE interrupt.
-	 *
-	 * GPE STS is a W1C register, which means:
-	 *
-	 * 1. Software can clear it without worrying about clearing the other
-	 *    GPEs' STS bits when the hardware sets them in parallel.
-	 *
-	 * 2. As long as software can ensure only clearing it when it is set,
-	 *    hardware won't set it in parallel.
-	 */
-	if (ec->gpe >= 0 && acpi_ec_gpe_status_set(ec))
-		acpi_clear_gpe(NULL, ec->gpe);
-
 	status = acpi_ec_read_status(ec);
 
 	/*
@@ -1287,6 +1272,22 @@ static void acpi_ec_handle_interrupt(struct acpi_ec *ec)
 	unsigned long flags;
 
 	spin_lock_irqsave(&ec->lock, flags);
+
+	/*
+	 * Clear GPE_STS upfront to allow subsequent hardware GPE_STS 0->1
+	 * changes to always trigger a GPE interrupt.
+	 *
+	 * GPE STS is a W1C register, which means:
+	 *
+	 * 1. Software can clear it without worrying about clearing the other
+	 *    GPEs' STS bits when the hardware sets them in parallel.
+	 *
+	 * 2. As long as software can ensure only clearing it when it is set,
+	 *    hardware won't set it in parallel.
+	 */
+	if (ec->gpe >= 0 && acpi_ec_gpe_status_set(ec))
+		acpi_clear_gpe(NULL, ec->gpe);
+
 	advance_transaction(ec, true);
 	spin_unlock_irqrestore(&ec->lock, flags);
 }
-- 
2.40.1


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>> > Signed-off-by: Jeremy Compostella <jeremy.compostella@intel.com>
>> > ---
>> >  drivers/acpi/ec.c | 2 +-
>> >  1 file changed, 1 insertion(+), 1 deletion(-)
>> >
>> > diff --git a/drivers/acpi/ec.c b/drivers/acpi/ec.c
>> > index 928899ab9502..42af09732238 100644
>> > --- a/drivers/acpi/ec.c
>> > +++ b/drivers/acpi/ec.c
>> > @@ -674,7 +674,7 @@ static void advance_transaction(struct acpi_ec *ec, bool interrupt)
>> >          * 2. As long as software can ensure only clearing it when it is set,
>> >          *    hardware won't set it in parallel.
>> >          */
>> > -       if (ec->gpe >= 0 && acpi_ec_gpe_status_set(ec))
>> > +       if (interrupt && ec->gpe >= 0 && acpi_ec_gpe_status_set(ec))
>> >                 acpi_clear_gpe(NULL, ec->gpe);
>> >
>> >         status = acpi_ec_read_status(ec);
>> > --