From 0e2a3ecdf8d3522f8621a2a474ec166d1f2ea0fc Mon Sep 17 00:00:00 2001
From: Kenneth Thompson <ken@flyvoly.com>
Date: Wed, 20 Oct 2021 19:55:54 -0700
Subject: [PATCH] drivers/can: Fix can_poll() POLLOUT calculation

can_poll() would indicate that there is no space in the TX FIFO if there is
already one element in the FIFO.
---
 drivers/can/can.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/can/can.c b/drivers/can/can.c
index 5d3f7c7c37..b10154af2f 100644
--- a/drivers/can/can.c
+++ b/drivers/can/can.c
@@ -1129,13 +1129,13 @@ static int can_poll(FAR struct file *filep, FAR struct pollfd *fds,
       while (ret < 0);
       dev->cd_ntxwaiters--;
 
-      ndx = dev->cd_xmit.tx_head + 1;
+      ndx = dev->cd_xmit.tx_tail + 1;
       if (ndx >= CONFIG_CAN_FIFOSIZE)
         {
           ndx = 0;
         }
 
-      if (ndx != dev->cd_xmit.tx_tail)
+      if (ndx != dev->cd_xmit.tx_head)
         {
           eventset |= fds->events & POLLOUT;
         }