[PATCH] serial: 8250_omap: Avoid FIFO corruption caused by MDR1 access

[Alexander Sverdlin <alexander.sverdlin@gmail.com>]

It has been observed that once per 300-1300 port openings the first
transmitted byte is being corrupted on AM3352 ("v" written to FIFO appeared
as "e" on the wire). It only happened if single byte has been transmitted
right after port open, which means, DMA is not used for this transfer and
the corruption never happened afterwards.

Therefore I've carefully re-read the MDR1 errata (link below), which says
"when accessing the MDR1 registers that causes a dummy under-run condition
that will freeze the UART in IrDA transmission. In UART mode, this may
corrupt the transferred data". Strictly speaking,
omap_8250_mdr1_errataset() performs a read access and if the value is the
same as should be written, exits without errata-recommended FIFO reset.

A brief check of the serial_omap_mdr1_errataset() from the competing
omap-serial driver showed it has no read access of MDR1. After removing the
read access from omap_8250_mdr1_errataset() the data corruption never
happened any more.

Link: https://www.ti.com/lit/er/sprz360i/sprz360i.pdf
Fixes: 61929cf0169d ("tty: serial: Add 8250-core based omap driver")
Cc: stable@vger.kernel.org
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
---
 drivers/tty/serial/8250/8250_omap.c | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_omap.c b/drivers/tty/serial/8250/8250_omap.c
index 562087df7d33..0cc6d35a0815 100644
--- a/drivers/tty/serial/8250/8250_omap.c
+++ b/drivers/tty/serial/8250/8250_omap.c
@@ -184,11 +184,6 @@ static void omap_8250_mdr1_errataset(struct uart_8250_port *up,
 				     struct omap8250_priv *priv)
 {
 	u8 timeout = 255;
-	u8 old_mdr1;
-
-	old_mdr1 = serial_in(up, UART_OMAP_MDR1);
-	if (old_mdr1 == priv->mdr1)
-		return;
 
 	serial_out(up, UART_OMAP_MDR1, priv->mdr1);
 	udelay(2);
-- 
2.29.2