On 10/2/24 9:49 AM, Uwe Kleine-König wrote:
On Wed, Oct 02, 2024 at 06:11:16AM +0200, Marek Vasut wrote:
When sending data using DMA at high baudrate (4 Mbdps in local test case) to
a device with small RX buffer which keeps asserting RTS after every received
byte, it is possible that the iMX UART driver would not recognize the falling
edge of RTS input signal and get stuck, unable to transmit any more data.
This condition happens when the following sequence of events occur:
- imx_uart_mctrl_check() is called at some point and takes a snapshot of UART
control signal status into sport->old_status using imx_uart_get_hwmctrl().
The RTSS/TIOCM_CTS bit is of interest here (*).
- DMA transfer occurs, the remote device asserts RTS signal after each byte.
The i.MX UART driver recognizes each such RTS signal change, raises an
interrupt with USR1 register RTSD bit set, which leads to invocation of
__imx_uart_rtsint(), which calls uart_handle_cts_change().
- If the RTS signal is deasserted, uart_handle_cts_change() clears
port->hw_stopped and unblocks the port for further data transfers.
- If the RTS is asserted, uart_handle_cts_change() sets port->hw_stopped
and blocks the port for further data transfers. This may occur as the
last interrupt of a transfer, which means port->hw_stopped remains set
and the port remains blocked (**).
- Any further data transfer attempts will trigger imx_uart_mctrl_check(),
which will read current status of UART control signals by calling
imx_uart_get_hwmctrl() (***) and compare it with sport->old_status .
- If current status differs from sport->old_status for RTS signal,
uart_handle_cts_change() is called and possibly unblocks the port
by clearing port->hw_stopped .
- If current status does not differ from sport->old_status for RTS
signal, no action occurs. This may occur in case prior snapshot (*)
was taken before any transfer so the RTS is deasserted, current
snapshot (***) was taken after a transfer and therefore RTS is
deasserted again, which means current status and sport->old_status
are identical. In case (**) triggered when RTS got asserted, and
made port->hw_stopped set, the port->hw_stopped will remain set
because no change on RTS line is recognized by this driver and
uart_handle_cts_change() is not called from here to unblock the
port->hw_stopped.
Update sport->old_status in __imx_uart_rtsint() accordingly to make
imx_uart_mctrl_check() detect such RTS change. Note that TIOCM_CAR
and TIOCM_RI bits in sport->old_status do not suffer from this problem.
Why is that? Just because these don't stop transmission?
If imx_uart_mctrl_check() does not detect the RTS asserted->deasserted
transition, it will never call uart_handle_cts_change(), which will
never clear port->hw_stopped and the port will remain stopped
indefinitely, and never be able to transmit more data AFTER this event
happens (port close/open will reset the flag too, but that is undesired
workaround).
Fixes: ceca629e0b48 ("[ARM] 2971/1: i.MX uart handle rts irq")
Signed-off-by: Marek Vasut <marex@xxxxxxx>
---
drivers/tty/serial/imx.c | 4 ++++
1 file changed, 4 insertions(+)
diff --git a/drivers/tty/serial/imx.c b/drivers/tty/serial/imx.c
index 67d4a72eda770..3ad7f42790ef9 100644
--- a/drivers/tty/serial/imx.c
+++ b/drivers/tty/serial/imx.c
@@ -762,6 +762,10 @@ static irqreturn_t __imx_uart_rtsint(int irq, void *dev_id)
imx_uart_writel(sport, USR1_RTSD, USR1);
usr1 = imx_uart_readl(sport, USR1) & USR1_RTSS;
+ if (usr1 & USR1_RTSS)
+ sport->old_status |= TIOCM_CTS;
+ else
+ sport->old_status &= ~TIOCM_CTS;
uart_handle_cts_change(&sport->port, usr1);
wake_up_interruptible(&sport->port.state->port.delta_msr_wait);
I didn't grab the whole picture, but I think this deserves a code
comment.
Added in V2.
Would it make sense to replace the current code in __imx_uart_rtsint by
a call to imx_uart_mctrl_check()?
No, the __imx_uart_rtsint() only handles RTS state change interrupt and
no other interrupts, so calling imx_uart_mctrl_check() would handle
other unrelated signals for no reason and make the interrupt handler
slower. I don't think this is an improvement.
