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

[Jassi Brar]

On Mon, Mar 23, 2026 at 12:14 AM Joonwon Kang <joonwonkang@xxxxxxxxxx> wrote:
>
> > 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 defined in the subsystem-internal mailbox.h so that
> > controller drivers within drivers/mailbox/ can reference it, but
> > it is not exposed to clients outside the subsystem.
>
> It sounds that it allows future controller drivers also to refer to the
> new sentinel pointer value.
>
Sentinel value is not the problem, active_req should have been hidden
from controllers. Which is actually respected by all controllers
except the tegra-hsp.c

> >
> > Fifteen in-tree callers send NULL (doorbell-style IPCs on Qualcomm,
> > Tegra, TI, Xilinx, i.MX, SCMI, and PCC platforms). All were
> > audited for regression:
> >
> >  - Most already work around the bug via knows_txdone=true with a
> >    manual mbox_client_txdone() call, making the framework's
> >    tracking irrelevant. These are unaffected.
> >
> >  - Poll-based callers (Xilinx zynqmp/r5) are strictly better off:
> >    the poll timer now correctly detects NULL-active channels
> >    instead of silently skipping them.
> >
> >  - irq-qcom-mpm.c was a pre-existing bug -- the only Qualcomm
> >    caller that omitted the knows_txdone + mbox_client_txdone()
> >    pattern. Fixed in a companion commit ("irqchip/qcom-mpm: Fix
> >    missing mailbox TX done acknowledgment").
> >
> >  - No caller sets both a tx_done callback and sends NULL, nor
> >    combines tx_block=true with NULL sends, so the newly reachable
> >    callback/completion paths are never exercised.
> >
> > Also update tegra-hsp's flush callback, which directly inspects
> > active_req to wait for the channel to drain: the old "!= NULL"
> > check becomes "!= MBOX_NO_MSG", otherwise flush spins until
> > timeout since the sentinel is non-NULL.
> >
> > The only tradeoff is that 'MBOX_NO_MSG' can not be used as a message
> > by clients.
>
> The other, but I guess more important, tradeoff is that future controller
> driver developers should now know that the pointer value of `->active->req`
> could be -1(== MBOX_NO_MSG) other than conventional pointer value(memory
> address, NULL, or error-encoded pointer value).
>
That should not be a concern. Controller drivers shouldn't peek into
mailbox internals and if they do they will know the sentinel value
being used.
For example, of the ~40 drivers, only tegra-hsp.c chose to (not had
to) use active_req and it relied on the sentinel value, which will now
be MBOX_NO_MSG.

Thanks
Jassi