can: mcp251xfd: mcp251xfd_get_tef_len(): fix length calculation

Commit b8e0ddd ("can: mcp251xfd: tef: prepare to workaround
broken TEF FIFO tail index erratum") introduced
mcp251xfd_get_tef_len() to get the number of unhandled transmit events
from the Transmit Event FIFO (TEF).

As the TEF has no head pointer, the driver uses the TX FIFO's tail
pointer instead, assuming that send frames are completed. However the
check for the TEF being full was not correct. This leads to the driver
stop working if the TEF is full.

Fix the TEF full check by assuming that if, from the driver's point of
view, there are no free TX buffers in the chip and the TX FIFO is
empty, all messages must have been sent and the TEF must therefore be
full.

Reported-by: Sven Schuchmann <[email protected]>
Closes: https://patch.msgid.link/FR3P281MB155216711EFF900AD9791B7ED9692@FR3P281MB1552.DEUP281.PROD.OUTLOOK.COM
Fixes: b8e0ddd ("can: mcp251xfd: tef: prepare to workaround broken TEF FIFO tail index erratum")
Tested-by: Sven Schuchmann <[email protected]>
Cc: [email protected]
Link: https://patch.msgid.link/20241104-mcp251xfd-fix-length-calculation-v3-1-608b6e7e2197@pengutronix.de
Signed-off-by: Marc Kleine-Budde <[email protected]>

Author: marckleinebudde

drivers/net/can/spi/mcp251xfd/mcp251xfd-tef.c

@@ -16,9 +16,9 @@
 
 #include "mcp251xfd.h"
 
-static inline bool mcp251xfd_tx_fifo_sta_full(u32 fifo_sta)
+static inline bool mcp251xfd_tx_fifo_sta_empty(u32 fifo_sta)
 {
-	return !(fifo_sta & MCP251XFD_REG_FIFOSTA_TFNRFNIF);
+	return fifo_sta & MCP251XFD_REG_FIFOSTA_TFERFFIF;
 }
 
 static inline int
@@ -122,7 +122,11 @@ mcp251xfd_get_tef_len(struct mcp251xfd_priv *priv, u8 *len_p)
 	if (err)
 		return err;
 
-	if (mcp251xfd_tx_fifo_sta_full(fifo_sta)) {
+	/* If the chip says the TX-FIFO is empty, but there are no TX
+	 * buffers free in the ring, we assume all have been sent.
+	 */
+	if (mcp251xfd_tx_fifo_sta_empty(fifo_sta) &&
+	    mcp251xfd_get_tx_free(tx_ring) == 0) {
 		*len_p = tx_ring->obj_num;
 		return 0;
 	}