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Subject[PATCH] Do not read IIR in serial8250_start_tx when UART_BUG_TXEN
The patch below has been lost several times (the last submission happened 
probably on [1]). I am currently aware of a real hardware which triggers
this problem quite reliably, so we should rather have that really fixed.

[1] http://lkml.org/lkml/2009/3/12/379




From: Ian Jackson <ian.jackson@eu.citrix.com>

Do not read IIR in serial8250_start_tx when UART_BUG_TXEN

Reading the IIR clears some oustanding interrupts so it is not safe.
Instead, simply transmit immediately if the buffer is empty without
regard to IIR.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Jiri Kosina <jkosina@suse.cz>

---
drivers/serial/8250.c | 8 +++-----
1 files changed, 3 insertions(+), 5 deletions(-)

diff --git a/drivers/serial/8250.c b/drivers/serial/8250.c
index 737b4c9..807042b 100644
--- a/drivers/serial/8250.c
+++ b/drivers/serial/8250.c
@@ -1339,14 +1339,12 @@ static void serial8250_start_tx(struct uart_port *port)
serial_out(up, UART_IER, up->ier);

if (up->bugs & UART_BUG_TXEN) {
- unsigned char lsr, iir;
+ unsigned char lsr;
lsr = serial_in(up, UART_LSR);
up->lsr_saved_flags |= lsr & LSR_SAVE_FLAGS;
- iir = serial_in(up, UART_IIR) & 0x0f;
if ((up->port.type == PORT_RM9000) ?
- (lsr & UART_LSR_THRE &&
- (iir == UART_IIR_NO_INT || iir == UART_IIR_THRI)) :
- (lsr & UART_LSR_TEMT && iir & UART_IIR_NO_INT))
+ (lsr & UART_LSR_THRE) :
+ (lsr & UART_LSR_TEMT))
transmit_chars(up);
}
}

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