linux-kernel - [PATCH 1/4] serial: 8250: Fall back to non-DMA Rx if IIR

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Message-Id: <20221107110708.58223-2-ilpo.jarvinen@linux.intel.com>
Date:   Mon,  7 Nov 2022 13:07:05 +0200
From:   Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>
Cc:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
        stable@...r.kernel.org,
        Srikanth Thokala <srikanth.thokala@...el.com>,
        Aman Kumar <aman.kumar@...el.com>
Subject: [PATCH 1/4] serial: 8250: Fall back to non-DMA Rx if IIR_RDI occurs

DW UART sometimes triggers IIR_RDI during DMA Rx when IIR_RX_TIMEOUT
should have been triggered instead. Since IIR_RDI has higher priority
than IIR_RX_TIMEOUT, this causes the Rx to hang into interrupt loop.
The problem seems to occur at least with some combinations of
small-sized transfers (I've reproduced the problem on Elkhart Lake PSE
UARTs).

If there's already an on-going Rx DMA and IIR_RDI triggers, fall
graciously back to non-DMA Rx. That is, behave as if IIR_RX_TIMEOUT had
occurred.

8250_omap already considers IIR_RDI similar to this change so its
nothing unheard of.

Fixes: 75df022b5f89 ("serial: 8250_dma: Fix RX handling")
Cc: <stable@...r.kernel.org>
Co-developed-by: Srikanth Thokala <srikanth.thokala@...el.com>
Signed-off-by: Srikanth Thokala <srikanth.thokala@...el.com>
Co-developed-by: Aman Kumar <aman.kumar@...el.com>
Signed-off-by: Aman Kumar <aman.kumar@...el.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
---
 drivers/tty/serial/8250/8250_port.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index fe8662cd9402..92dd18716169 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -1897,6 +1897,10 @@ EXPORT_SYMBOL_GPL(serial8250_modem_status);
 static bool handle_rx_dma(struct uart_8250_port *up, unsigned int iir)
 {
 	switch (iir & 0x3f) {
+	case UART_IIR_RDI:
+		if (!up->dma->rx_running)
+			break;
+		fallthrough;
 	case UART_IIR_RX_TIMEOUT:
 		serial8250_rx_dma_flush(up);
 		fallthrough;
-- 
2.30.2