Re: [PATCH] RFC: mailbox: Fix NULL message support in mbox_send_message()

[Jassi Brar]

On Tue, Mar 10, 2026 at 6:52 PM Doug Anderson <dianders@xxxxxxxxxxxx> wrote:
>
> Hi,
>
> On Tue, Mar 10, 2026 at 4:46 PM <jassisinghbrar@xxxxxxxxx> wrote:
> >
> > From: Jassi Brar <jassisinghbrar@xxxxxxxxx>
> >
> > The active_req field serves double duty as both the "is a TX in
> > flight" flag (NULL means idle) and the storage for the in-flight
> > message pointer. When a client sends NULL via mbox_send_message(),
> > active_req is set to NULL, which the framework misinterprets as
> > "no active request." This breaks the TX state machine by:
> >
> >  - tx_tick() short-circuits on (!mssg), skipping the tx_done
> >    callback and the tx_complete completion
> >  - txdone_hrtimer() skips the channel entirely since active_req
> >    is NULL, so poll-based TX-done detection never fires.
> >
> > Fix this by introducing a MBOX_NO_MSG sentinel value that means
> > "no active request," freeing NULL to be valid message data. The
> > sentinel is internal to the mailbox core and is never exposed to
> > controller drivers or clients.
> >
> > The only tradeoff is that 'MBOX_NO_MSG' can not be used as a message
> > by clients.
> >
> > Signed-off-by: Jassi Brar <jassisinghbrar@xxxxxxxxx>
> > ---
> >  drivers/mailbox/mailbox.c | 15 +++++++++------
> >  1 file changed, 9 insertions(+), 6 deletions(-)
>
> While I can certainly be corrected, I suspect this patch will break a
> lot of users.
>
> My analysis of people that were passing NULL mbox messages is that
> they _wanted_ the current behavior. They didn't want NULL messages to
> be queued up as would happen with this patch. Instead, they just
> wanted to immediately ring the doorbell again to signal an interrupt
> to the other side.
>
They won't be queued up because they call mbox_client_txdone()
immediately after mbox_send_message()
>From my analysis of the 14 clients, there was just one
(irq-qcom-mpm.c) that was not doing it by oversight, and I submitted a
patch.

Can you please point me to the driver you are concerned about? Perhaps
I am overlooking something.

Regards,
Jassi