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

Sudeep Holla
2025-03-13T15:09:52Z

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@xsightlabs.com>
> > Reviewed-by: Huisong Li <lihuisong@huawei.com>
> > Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
>
> Tested-by: Robbie King <robbiek@xsightlabs.com>
>

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

--
Regards,
Sudeep