[PATCH v3 12/21] serial: sh: Fix error handling

[Paul Barker]

The current SCIF error handling is broken for the RZ/G2L. After a break
condition has been triggered, the current code is unable to clear the
error and serial port output never resumes.

The RZ/G2L datasheet says that most error conditions are cleared by
resetting the relevant error bits in the FSR & LSR registers to zero.
To clear framing errors, the invalid data also needs to be read out of
the receive FIFO.

After reviewing datasheets for RZ/G2{H,M,N,E}, R-Car Gen4, R-Car Gen3
and even SH7751 SoCs, it's clear that this is the way to clear errors
for all of these SoCs.

While we're here, annotate the handle_error() function with a couple of
comments as the reads and writes themselves don't immediately make it
clear what we're doing.

Signed-off-by: Paul Barker <paul.barker.ct at bp.renesas.com>
Tested-by: Chris Paterson <chris.paterson2 at renesas.com>
	# HiHope RZ/G2M board
Tested-by: Marek Vasut <marek.vasut+renesas at mailbox.org>
	# R-Car H3 Salvator-XS
---
v2->v3:
  * Added Chris' and Marek's Tested-by.
v1->v2:
  * New patch after discussion with Marek & further investigation.

I've not put a `Fixes` tag on this, the error handling has been writing
ones to clear errors since at least 2007 so maybe it was originally
right for some board. It doesn't seem to apply to any of the current
users of the SCIF driver though.

 drivers/serial/serial_sh.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/drivers/serial/serial_sh.c b/drivers/serial/serial_sh.c
index 5e543dbf3d58..2b650d458e71 100644
--- a/drivers/serial/serial_sh.c
+++ b/drivers/serial/serial_sh.c
@@ -79,10 +79,22 @@ sh_serial_setbrg_generic(struct uart_port *port, int clk, int baudrate)
 
 static void handle_error(struct uart_port *port)
 {
-	sci_in(port, SCxSR);
-	sci_out(port, SCxSR, SCxSR_ERROR_CLEAR(port));
+	/*
+	 * Most errors are cleared by resetting the relevant error bits to zero
+	 * in the FSR & LSR registers. For each register, a read followed by a
+	 * write is needed according to the relevant datasheets.
+	 */
+	unsigned short status = sci_in(port, SCxSR);
+	sci_out(port, SCxSR, status & ~SCIF_ERRORS);
 	sci_in(port, SCLSR);
 	sci_out(port, SCLSR, 0x00);
+
+	/*
+	 * To clear framing errors, we also need to read and discard a
+	 * character.
+	 */
+	if (status & SCIF_FER)
+		sci_in(port, SCxRDR);
 }
 
 static int serial_raw_putc(struct uart_port *port, const char c)
-- 
2.39.2