[1/2] serial: 8250: Don't service RX FIFO if interrupts are disabled
diff mbox

Message ID 20180208125542.15649-2-vigneshr@ti.com
State New
Headers show

Commit Message

Vignesh Raghavendra Feb. 8, 2018, 12:55 p.m. UTC
Currently, data in RX FIFO is read based on UART_LSR register state even
if RDI and RLSI interrupts are disabled in UART_IER register.
This is because when IRQ handler is called due to TX FIFO empty event,
RX FIFO is serviced based on UART_LSR register status instead of
UART_IIR status. This defeats the purpose of disabling UART RX
FIFO interrupts during throttling(see, omap_8250_throttle()) as IRQ
handler continues to drain UART RX FIFO resulting in overflow of buffer
at tty layer.
Fix this by making sure that driver drains UART RX FIFO only when
UART_IIR_RDI is set along with UART_LSR_BI or UART_LSR_DR bits.

Signed-off-by: Vignesh R <vigneshr@ti.com>
---
 drivers/tty/serial/8250/8250_port.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

Comments

Andy Shevchenko Feb. 8, 2018, 3:16 p.m. UTC | #1
On Thu, Feb 8, 2018 at 2:55 PM, Vignesh R <vigneshr@ti.com> wrote:
> Currently, data in RX FIFO is read based on UART_LSR register state even
> if RDI and RLSI interrupts are disabled in UART_IER register.
> This is because when IRQ handler is called due to TX FIFO empty event,
> RX FIFO is serviced based on UART_LSR register status instead of
> UART_IIR status. This defeats the purpose of disabling UART RX
> FIFO interrupts during throttling(see, omap_8250_throttle()) as IRQ
> handler continues to drain UART RX FIFO resulting in overflow of buffer
> at tty layer.
> Fix this by making sure that driver drains UART RX FIFO only when
> UART_IIR_RDI is set along with UART_LSR_BI or UART_LSR_DR bits.
>
> Signed-off-by: Vignesh R <vigneshr@ti.com>

> -       if (status & (UART_LSR_DR | UART_LSR_BI)) {
> +       if (status & (UART_LSR_DR | UART_LSR_BI) &&
> +           iir & UART_IIR_RDI) {

>                 if (!up->dma || handle_rx_dma(up, iir))

handle_rx_dma() checks for IRQ status as well.

But for now it seems we are on safe side since checks are done versus
IRQ status with bit 2 set, meaning that iir & RDI will be true.

>                         status = serial8250_rx_chars(up, status);
>         }

Anyway, thanks for the patch, though I need some time to test it on
non-OMAP hardware with DMA enabled.
Vignesh Raghavendra Feb. 8, 2018, 5:15 p.m. UTC | #2
On 08-Feb-18 8:46 PM, Andy Shevchenko wrote:
> On Thu, Feb 8, 2018 at 2:55 PM, Vignesh R <vigneshr@ti.com> wrote:
>> Currently, data in RX FIFO is read based on UART_LSR register state even
>> if RDI and RLSI interrupts are disabled in UART_IER register.
>> This is because when IRQ handler is called due to TX FIFO empty event,
>> RX FIFO is serviced based on UART_LSR register status instead of
>> UART_IIR status. This defeats the purpose of disabling UART RX
>> FIFO interrupts during throttling(see, omap_8250_throttle()) as IRQ
>> handler continues to drain UART RX FIFO resulting in overflow of buffer
>> at tty layer.
>> Fix this by making sure that driver drains UART RX FIFO only when
>> UART_IIR_RDI is set along with UART_LSR_BI or UART_LSR_DR bits.
>>
>> Signed-off-by: Vignesh R <vigneshr@ti.com>
> 
>> -       if (status & (UART_LSR_DR | UART_LSR_BI)) {
>> +       if (status & (UART_LSR_DR | UART_LSR_BI) &&
>> +           iir & UART_IIR_RDI) {
> 
>>                 if (!up->dma || handle_rx_dma(up, iir))
> 
> handle_rx_dma() checks for IRQ status as well.
> 
> But for now it seems we are on safe side since checks are done versus
> IRQ status with bit 2 set, meaning that iir & RDI will be true.
> 
>>                         status = serial8250_rx_chars(up, status);
>>         }
> 
> Anyway, thanks for the patch, though I need some time to test it on
> non-OMAP hardware with DMA enabled.

This patch is needed even when DMA is not enabled.
It would be great if you could test this. But, I don't see any other
8250 drivers apart from 8250_omap.c implementing
.throttle()/.unthrottle() callbacks.

Regards
Vignesh
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Patch
diff mbox

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 1328c7e70108..ffbb955d1c06 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -1854,7 +1854,8 @@  int serial8250_handle_irq(struct uart_port *port, unsigned int iir)
 
 	status = serial_port_in(port, UART_LSR);
 
-	if (status & (UART_LSR_DR | UART_LSR_BI)) {
+	if (status & (UART_LSR_DR | UART_LSR_BI) &&
+	    iir & UART_IIR_RDI) {
 		if (!up->dma || handle_rx_dma(up, iir))
 			status = serial8250_rx_chars(up, status);
 	}