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

[Doug Anderson]

Hi,

On Tue, Mar 10, 2026 at 8:42 PM Jassi Brar <jassisinghbrar@xxxxxxxxx> wrote:
>
> > > > In my case, I have a mailbox driver that's currently downstream
> > > > (though I hope to change that). My mailbox controller has an interrupt
> > > > for txdone, so mailbox clients _shouldn't_ call mbox_client_txdone().
> > > > Some clients of this mailbox client want the txdone interrupt, but
> > > > some clients of it just care about sending doorbells.
> > > >
> > > So imx-dsp.c like? Please let me know what is lacking in the core to
> > > fully support that. Happy to look into it.
> >
> > I know that with my downstream mailbox client, if I let NULL messages
> > queue up I end up with a queue of a dozen or so NULL messages. The
> > downstream client is really taking advantage (AKA abusing) the core's
> > current NULL behavior. It truly does want the "mbox_ring_doorbell"
> > concept of just making sure the doorbell is asserted and returning
> > immediately. If the core changes to start queuing NULL messages, we'll
> > have to figure out some sort of workaround...
> >
> You said your controller has an irq raised for the doorbell sent (?).

It has an interrupt for when the remote side receives the doorbell
(when it clears the IRQ on its side).

> If so, your clients should want to wait for that confirmation (the
> controller driver would tick via mbox_chan_txdone).

Ah, I think I see what you're saying. Functionally, I think it should
work. Pseudocode for the client:

def ring_doorbell():
  if not previous_doorbell_acked:
    another_doorbell_pending = true
  else:
    previous_doorbell_acked = false;
    mbox_send_message(NULL)

def tx_done():
  if another_doorbell_pending:
    another_doorbell_pending = false
    mbox_send_message(NULL)
  else:
    previous_doorbell_acked = true;

I think that will ensure the other side always gets at least one
future interrupt when the doorbell is rung.

I guess another option would be for the mailbox controller to issue
"tx_done" immediately for NULL messages. With no data to transmit, I
guess one could say that as soon as the interrupt is raised that the
data is "transmitted", so maybe this would be OK too?

-Doug