From bb07f4c93e62a310afd9aa6250744818d7f3de7d Mon Sep 17 00:00:00 2001 From: Alexander Sverdlin Date: Thu, 10 Dec 2020 06:52:57 +0100 Subject: [PATCH] serial: 8250_omap: Avoid FIFO corruption caused by MDR1 access commit d96f04d347e4011977abdbb4da5d8f303ebd26f8 upstream. 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 Link: https://lore.kernel.org/r/20201210055257.1053028-1-alexander.sverdlin@gmail.com Signed-off-by: Greg Kroah-Hartman --- 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 f2c6d9d3bb28..efe793a2fc65 100644 --- a/drivers/tty/serial/8250/8250_omap.c +++ b/drivers/tty/serial/8250/8250_omap.c @@ -170,11 +170,6 @@ 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);