linux-kernel - Re: [PATCH v2 02/13] mailbox: pcc: Always clear the platform ack interrupt first

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Message-ID: <Z9L1PD8lDjwM7L3l@bogus>
Date: Thu, 13 Mar 2025 15:09:48 +0000
From: Sudeep Holla <sudeep.holla@....com>
To: Robbie King <robbiek@...ghtlabs.com>
Cc: <linux-acpi@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	Jassi Brar <jassisinghbrar@...il.com>,
	Sudeep Holla <sudeep.holla@....com>,
	Huisong Li <lihuisong@...wei.com>,
	Adam Young <admiyo@...amperecomputing.com>
Subject: Re: [PATCH v2 02/13] mailbox: pcc: Always clear the platform ack
 interrupt first

On Thu, Mar 13, 2025 at 11:08:04AM -0400, Robbie King wrote:
> On 3/5/2025 11:38 AM, Sudeep Holla wrote:
> > The PCC mailbox interrupt handler (pcc_mbox_irq()) currently checks
> > for command completion flags and any error status before clearing the
> > interrupt.
> > 
> > The below sequence highlights an issue in the handling of PCC mailbox
> > interrupts, specifically when dealing with doorbell notifications and
> > acknowledgment between the OSPM and the platform where type3 and type4
> > channels are sharing the interrupt.
> > 
> > -------------------------------------------------------------------------
> > | T |       Platform Firmware         |    OSPM/Linux PCC driver        |
> > |---|---------------------------------|---------------------------------|
> > | 1 |                                 | Build message in shmem          |
> > | 2 |                                 | Ring Type3 chan doorbell        |
> > | 3 | Receives the doorbell interrupt |                                 |
> > | 4 | Process the message from OSPM   |                                 |
> > | 5 | Build response for the message  |                                 |
> > | 6 | Ring Platform ACK interrupt on  |                                 |
> > |   |  Type3 chan to OSPM             | Received the interrupt          |
> > | 7 | Build Notification in Type4 Chan|                                 |
> > | 8 |                                 | Start processing interrupt in   |
> > |   |                                 |  pcc_mbox_irq() handler         |
> > | 9 |                                 | Enter PCC handler for Type4 chan|
> > |10 |                                 | Check command complete cleared  |
> > |11 |                                 | Read the notification           |
> > |12 |                                 | Clear Platform ACK interrupt    |
> > |   | No effect from the previous step yet as the Platform ACK          |
> > |   |  interrupt has not yet been triggered for this channel            |
> > |13 | Ring Platform ACK interrupt on  |                                 |
> > |   | Type4 chan to OSPM              |                                 |
> > |14 |                                 | Enter PCC handler for Type3 chan|
> > |15 |                                 | Command complete is set.        |
> > |16 |                                 | Read the response.              |
> > |17 |                                 | Clear Platform ACK interrupt    |
> > |18 |                                 | Leave PCC handler for Type3     |
> > |19 |                                 | Leave pcc_mbox_irq() handler    |
> > |20 |                                 | Re-enter pcc_mbox_irq() handler |
> > |21 |                                 | Enter PCC handler for Type4 chan|
> > |22 |                                 | Leave PCC handler for Type4 chan|
> > |23 |                                 | Enter PCC handler for Type3 chan|
> > |24 |                                 | Leave PCC handler for Type3 chan|
> > |25 |                                 | Leave pcc_mbox_irq() handler    |
> > -------------------------------------------------------------------------
> > 
> > The key issue occurs when OSPM tries to acknowledge platform ack
> > interrupt for a notification which is ready to be read and processed
> > but the interrupt itself is not yet triggered by the platform.
> > 
> > This ineffective acknowledgment leads to an issue later in time where
> > the interrupt remains pending as we exit the interrupt handler without
> > clearing the platform ack interrupt as there is no pending response or
> > notification. The interrupt acknowledgment order is incorrect.
> > 
> > To resolve this issue, the platform acknowledgment interrupt should
> > always be cleared before processing the interrupt for any notifications
> > or response.
> > 
> > Reported-by: Robbie King <robbiek@...ghtlabs.com>
> > Reviewed-by: Huisong Li <lihuisong@...wei.com>
> > Signed-off-by: Sudeep Holla <sudeep.holla@....com>
> 
> Tested-by: Robbie King <robbiek@...ghtlabs.com>
> 

Thanks Robbie for reporting the issue and testing the fix. Much appreciated!

-- 
Regards,
Sudeep