RE: [PATCH 03/14] serial: tegra: avoid reg access when clk disabled

-----Original Message-----
From: linux-tegra-owner@vger.kernel.org <linux-tegra- 
owner@vger.kernel.org> On Behalf Of Thierry Reding
Sent: Tuesday, August 13, 2019 3:16 PM
To: Krishna Yarlagadda <redacted>
Cc: gregkh@linuxfoundation.org; robh+dt@kernel.org; 
mark.rutland@arm.com; Jonathan Hunter [off-list ref]; Laxman 
Dewangan [off-list ref]; jslaby@suse.com; linux- 
serial@vger.kernel.org; devicetree@vger.kernel.org; linux- 
tegra@vger.kernel.org; linux-kernel@vger.kernel.org; Ahung Cheng 
[off-list ref]; Shardar Mohammed [off-list ref]
Subject: Re: [PATCH 03/14] serial: tegra: avoid reg access when clk 
disabled

On Mon, Aug 12, 2019 at 04:58:12PM +0530, Krishna Yarlagadda wrote:

quoted

From: Ahung Cheng <redacted>

This avoids two race conditions from the UART shutdown sequence both 
leading to 'Machine check error in AXI2APB' and kernel oops.

One was that the clock was disabled before the DMA was terminated 
making it possible for the DMA callbacks to be called after the 
clock was disabled. These callbacks could write to the UART 
registers causing timeout.

The second was that the clock was disabled before the UART was 
completely flagged as closed. This is done after the shutdown is 
called and a new write could be started after the clock was disabled.
tegra_uart_start_pio_tx could be called causing timeout.

Given that the baud rate is reset at the end of shutdown sequence, 
this fix is to examine the baud rate to avoid register access from 
both race conditions.

Besides, terminate the DMA before disabling the clock.

Signed-off-by: Ahung Cheng <redacted>
Signed-off-by: Shardar Mohammed <redacted>
Signed-off-by: Krishna Yarlagadda <redacted>
---
 drivers/tty/serial/serial-tegra.c | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/drivers/tty/serial/serial-tegra.c

b/drivers/tty/serial/serial-tegra.c
index 93d299e..d908465 100644

--- a/drivers/tty/serial/serial-tegra.c
+++ b/drivers/tty/serial/serial-tegra.c

@@ -126,6 +126,8 @@ struct tegra_uart_port {

 static void tegra_uart_start_next_tx(struct tegra_uart_port *tup); 

static int tegra_uart_start_rx_dma(struct tegra_uart_port *tup);
+static void tegra_uart_dma_channel_free(struct tegra_uart_port *tup,
+					bool dma_to_memory);

 static inline unsigned long tegra_uart_read(struct tegra_uart_port *tup,
 		unsigned long reg)

@@ -458,6 +460,9 @@ static void tegra_uart_start_next_tx(struct

tegra_uart_port *tup)

quoted

 	unsigned long count;
 	struct circ_buf *xmit = &tup->uport.state->xmit;

+	if (WARN_ON(!tup->current_baud))
+		return;

Are the race conditions that you are describing something which can be 
triggered by the user? If so, it's not a good idea to use a WARN_ON, 
because that could lead to some userspace spamming the log with these, 
potentially on purpose.

Thierry

These are triggered by user events. I will remove WARN_ON

 KY

quoted

+
 	tail = (unsigned long)&xmit->buf[xmit->tail];
 	count = CIRC_CNT_TO_END(xmit->head, xmit->tail,

UART_XMIT_SIZE);

quoted

 	if (!count)

@@ -829,6 +834,12 @@ static void tegra_uart_hw_deinit(struct

tegra_uart_port *tup)

quoted

 	tup->current_baud = 0;
 	spin_unlock_irqrestore(&tup->uport.lock, flags);

+	tup->rx_in_progress = 0;
+	tup->tx_in_progress = 0;
+
+	tegra_uart_dma_channel_free(tup, true);
+	tegra_uart_dma_channel_free(tup, false);
+
 	clk_disable_unprepare(tup->uart_clk);
 }

@@ -1066,12 +1077,6 @@ static void tegra_uart_shutdown(struct

uart_port *u)

quoted

 	struct tegra_uart_port *tup = to_tegra_uport(u);

 	tegra_uart_hw_deinit(tup);
-
-	tup->rx_in_progress = 0;
-	tup->tx_in_progress = 0;
-
-	tegra_uart_dma_channel_free(tup, true);
-	tegra_uart_dma_channel_free(tup, false);
 	free_irq(u->irq, tup);
 }

--
2.7.4