mailbox: Fix NULL message support in mbox_send_message()

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.

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.

Reported-by: Joonwon Kang <joonwonkang@google.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>

diff --git a/drivers/mailbox/mailbox.c b/drivers/mailbox/mailbox.c
index 13de3d047853e2cb50b2a56ada475c2e6bc11a20..138ffbcd4fde33befde3a8c45d6081da623922c1 100644 (file)
--- a/drivers/mailbox/mailbox.c
+++ b/drivers/mailbox/mailbox.c
@@ -50,7 +50,7 @@ static void msg_submit(struct mbox_chan *chan)
        int err = -EBUSY;
 
        scoped_guard(spinlock_irqsave, &chan->lock) {
-               if (!chan->msg_count || chan->active_req)
+               if (!chan->msg_count || chan->active_req != MBOX_NO_MSG)
                        break;
 
                count = chan->msg_count;
@@ -85,13 +85,13 @@ static void tx_tick(struct mbox_chan *chan, int r)
 
        scoped_guard(spinlock_irqsave, &chan->lock) {
                mssg = chan->active_req;
-               chan->active_req = NULL;
+               chan->active_req = MBOX_NO_MSG;
        }
 
        /* Submit next message */
        msg_submit(chan);
 
-       if (!mssg)
+       if (mssg == MBOX_NO_MSG)
                return;
 
        /* Notify the client */
@@ -112,7 +112,7 @@ static enum hrtimer_restart txdone_hrtimer(struct hrtimer *hrtimer)
        for (i = 0; i < mbox->num_chans; i++) {
                struct mbox_chan *chan = &mbox->chans[i];
 
-               if (chan->active_req && chan->cl) {
+               if (chan->active_req != MBOX_NO_MSG && chan->cl) {
                        txdone = chan->mbox->ops->last_tx_done(chan);
                        if (txdone)
                                tx_tick(chan, 0);
@@ -267,7 +267,7 @@ int mbox_send_message(struct mbox_chan *chan, void *mssg)
 {
        int t;
 
-       if (!chan || !chan->cl)
+       if (!chan || !chan->cl || mssg == MBOX_NO_MSG)
                return -EINVAL;
 
        t = add_to_rbuf(chan, mssg);
@@ -340,7 +340,7 @@ static int __mbox_bind_client(struct mbox_chan *chan, struct mbox_client *cl)
        scoped_guard(spinlock_irqsave, &chan->lock) {
                chan->msg_free = 0;
                chan->msg_count = 0;
-               chan->active_req = NULL;
+               chan->active_req = MBOX_NO_MSG;
                chan->cl = cl;
                init_completion(&chan->tx_complete);
 
@@ -498,7 +498,7 @@ void mbox_free_channel(struct mbox_chan *chan)
        /* The queued TX requests are simply aborted, no callbacks are made */
        scoped_guard(spinlock_irqsave, &chan->lock) {
                chan->cl = NULL;
-               chan->active_req = NULL;
+               chan->active_req = MBOX_NO_MSG;
                if (chan->txdone_method == TXDONE_BY_ACK)
                        chan->txdone_method = TXDONE_BY_POLL;
        }
@@ -553,6 +553,7 @@ int mbox_controller_register(struct mbox_controller *mbox)
 
                chan->cl = NULL;
                chan->mbox = mbox;
+               chan->active_req = MBOX_NO_MSG;
                chan->txdone_method = txdone;
                spin_lock_init(&chan->lock);
        }

diff --git a/drivers/mailbox/tegra-hsp.c b/drivers/mailbox/tegra-hsp.c
index 2231050bb5a9b61d24396c7642fcdd47aa965085..7b1e1b83ea29848dbddcd7c8f0ce4dc34038311d 100644 (file)
--- a/drivers/mailbox/tegra-hsp.c
+++ b/drivers/mailbox/tegra-hsp.c
@@ -495,7 +495,7 @@ static int tegra_hsp_mailbox_flush(struct mbox_chan *chan,
                        mbox_chan_txdone(chan, 0);
 
                        /* Wait until channel is empty */
-                       if (chan->active_req != NULL)
+                       if (chan->active_req != MBOX_NO_MSG)
                                continue;
 
                        return 0;

diff --git a/include/linux/mailbox_controller.h b/include/linux/mailbox_controller.h
index 16fef421c30cf430a37f84bbaff05735d2d0c36a..e3896b08f22e54169e8f24db7e2bb9c3cbe98420 100644 (file)
--- a/include/linux/mailbox_controller.h
+++ b/include/linux/mailbox_controller.h
@@ -12,6 +12,9 @@
 
 struct mbox_chan;
 
+/* Sentinel value distinguishing "no active request" from "NULL message data" */
+#define MBOX_NO_MSG    ((void *)-1)
+
 #define TXDONE_BY_IRQ  BIT(0) /* controller has remote RTR irq */
 #define TXDONE_BY_POLL BIT(1) /* controller can read status of last TX */
 #define TXDONE_BY_ACK  BIT(2) /* S/W ACK received by Client ticks the TX */