[RFC v2 02/18] mailbox: Add an API for flushing the FIFO

[Vaibhav Bedia]

On AM33XX, the mailbox module between the MPU and the
WKUP-M3 co-processor facilitates a one-way communication.
MPU uses the assigned mailbox sub-module to issue the
interrupt to the WKUP-M3 co-processor which then goes
and reads the the IPC data from registers in the control
module.

WKUP-M3 is in the L4_WKUP and does not have any access to
the mailbox module. Due to this limitation, the MPU is
completely responsible for FIFO maintenance and interrupt
generation. MPU needs to ensure that the FIFO does not
overflow by reading back the assigned mailbox sub-module.

This patch adds an API in the mailbox code which the MPU
can use to empty the FIFO by issuing a readback command.

Signed-off-by: Vaibhav Bedia <vaibhav.bedia at ti.com>
Cc: Santosh Shilimkar <santosh.shilimkar at ti.com>
---
Note: This patch which will be slightly reworked once the mailbox
driver changes are finalized.

 drivers/mailbox/mailbox-omap2.c |   19 +++++++++++++++++++
 drivers/mailbox/mailbox.c       |   36 ++++++++++++++++++++++++++++++++++++
 drivers/mailbox/mailbox.h       |    3 +++
 include/linux/mailbox.h         |    1 +
 4 files changed, 59 insertions(+), 0 deletions(-)

diff --git a/drivers/mailbox/mailbox-omap2.c b/drivers/mailbox/mailbox-omap2.c
index 6d61159..c732be1 100644
--- a/drivers/mailbox/mailbox-omap2.c
+++ b/drivers/mailbox/mailbox-omap2.c
@@ -125,6 +125,23 @@ static int omap2_mbox_fifo_full(struct mailbox *mbox)
 	return mbox_read_reg(fifo->fifo_stat);
 }
 
+static int omap2_mbox_fifo_needs_flush(struct mailbox *mbox)
+{
+	struct omap_mbox2_priv *p = mbox->priv;
+	struct omap_mbox2_fifo *fifo = &p->tx_fifo;
+
+	return mbox_read_reg(fifo->msg_stat);
+}
+
+static void omap2_mbox_fifo_readback(struct mailbox *mbox,
+			struct mailbox_msg *msg)
+{
+	struct omap_mbox2_priv *p = mbox->priv;
+	struct omap_mbox2_fifo *fifo = &p->tx_fifo;
+
+	msg->header = mbox_read_reg(fifo->msg);
+}
+
 static int ompa2_mbox_poll_for_space(struct mailbox *mbox)
 {
 	if (omap2_mbox_fifo_full(mbox))
@@ -221,6 +238,8 @@ static struct mailbox_ops omap2_mbox_ops = {
 	.read           = omap2_mbox_fifo_read,
 	.write          = omap2_mbox_fifo_write,
 	.empty          = omap2_mbox_fifo_empty,
+	.fifo_needs_flush	= omap2_mbox_fifo_needs_flush,
+	.fifo_readback		= omap2_mbox_fifo_readback,
 	.poll_for_space = ompa2_mbox_poll_for_space,
 	.enable_irq     = omap2_mbox_enable_irq,
 	.disable_irq    = omap2_mbox_disable_irq,
diff --git a/drivers/mailbox/mailbox.c b/drivers/mailbox/mailbox.c
index 2f50226..92c9f68 100644
--- a/drivers/mailbox/mailbox.c
+++ b/drivers/mailbox/mailbox.c
@@ -57,6 +57,15 @@ static inline int mbox_empty(struct mailbox *mbox)
 {
 	return mbox->ops->empty(mbox);
 }
+static inline int mbox_fifo_needs_flush(struct mailbox *mbox)
+{
+	return mbox->ops->fifo_needs_flush(mbox);
+}
+static inline void mbox_fifo_readback(struct mailbox *mbox,
+				struct mailbox_msg *msg)
+{
+	mbox->ops->fifo_readback(mbox, msg);
+}
 
 /* Mailbox IRQ handle functions */
 static inline void ack_mbox_irq(struct mailbox *mbox, mailbox_irq_t irq)
@@ -110,6 +119,33 @@ out:
 }
 EXPORT_SYMBOL(mailbox_msg_send);
 
+/*
+ * Flush the Rx FIFO by reading back the messages
+ * Since the normal expectation is that the Rx will do the
+ * reading, add a debug message to indicate if we really flush
+ *
+ * Returns the no. of messages read back
+ */
+int mailbox_rx_flush(struct mailbox *mbox)
+{
+	int ret = 0;
+	struct mailbox_msg readback_msg;
+
+	while (mbox_fifo_needs_flush(mbox)) {
+		mbox_fifo_readback(mbox, &readback_msg);
+		ret++;
+	}
+
+	if (ret) {
+		/* no more messages in the fifo. clear IRQ source. */
+		ack_mbox_irq(mbox, IRQ_RX);
+		pr_debug("Flushed %s Rx FIFO by reading back\n", mbox->name);
+	}
+
+	return ret;
+}
+EXPORT_SYMBOL(mailbox_rx_flush);
+
 #define TRANSFER_TIMEOUT 30000 /* Becomes ~3s timeout */
 
 static struct mailbox_msg no_irq_msg_res;
diff --git a/drivers/mailbox/mailbox.h b/drivers/mailbox/mailbox.h
index f103194..b841ae1 100644
--- a/drivers/mailbox/mailbox.h
+++ b/drivers/mailbox/mailbox.h
@@ -24,6 +24,9 @@ struct mailbox_ops {
 	int             (*write)(struct mailbox *mbox, struct mailbox_msg *msg);
 	int             (*empty)(struct mailbox *mbox);
 	int             (*poll_for_space)(struct mailbox *mbox);
+	int		(*fifo_needs_flush)(struct mailbox *mbox);
+	void		(*fifo_readback)(struct mailbox *mbox,
+				struct mailbox_msg *msg);
 	/* irq */
 	void            (*enable_irq)(struct mailbox *mbox,
 			mailbox_irq_t irq);
diff --git a/include/linux/mailbox.h b/include/linux/mailbox.h
index 3f9ec1e..705def9 100644
--- a/include/linux/mailbox.h
+++ b/include/linux/mailbox.h
@@ -22,6 +22,7 @@ struct mailbox_msg {
 #define MAILBOX_FILL_HEADER_MSG(_msg, _header) \
 	MAILBOX_FILL_MSG(_msg, _header, NULL, 0);
 
+int mailbox_rx_flush(struct mailbox *);
 int mailbox_msg_send(struct mailbox *, struct mailbox_msg *msg);
 struct mailbox_msg *mailbox_msg_send_receive_no_irq(struct mailbox *,
 		struct mailbox_msg *msg);
-- 
1.7.0.4