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Re: [PATCH v2 2/2] xen/ns16550: drain TX before reinitializing
- To: Zhang Zheng <zhangzheng@xxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- From: Baptiste Le Duc <baptiste.le-duc@xxxxxxxxxx>
- Date: Sun, 11 Oct 2026 00:13:01 +0200
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- Cc: Alistair Francis <alistair.francis@xxxxxxx>, Connor Davis <connojdavis@xxxxxxxxx>, Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Julien Grall <julien@xxxxxxx>, Roger Pau Monné <roger@xxxxxxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>
- Delivery-date: Sat, 10 Oct 2026 22:13:12 +0000
- Feedback-id: default:8631fc262581453bbf619ec5b2062170:Sweego
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
For any reasons, it seems this patch has not been received by ML server,
I couldn't find him here:
https://lore.kernel.org/xen-devel/20261010031406.2057735-1-zhangzheng@xxxxxxxxxxx/T/#t
On 10/10/26 5:21 AM, Zhang Zheng wrote:
An SBI console call may return before the UART finishes transmitting.
Wait for UART_LSR_TEMT before ns16550 reprograms the UART and clears its
FIFO, preserving pending early-console output and legacy SBI v0.1 support.
Suggested-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
Link:
https://lore.kernel.org/xen-devel/8836d1e3-77c8-4447-b1ef-132620a71441@xxxxxxxxx/
Signed-off-by: Zhang Zheng <zhangzheng@xxxxxxxxxxx>
---
xen/drivers/char/ns16550.c | 4 ++++
1 file changed, 4 insertions(+)
diff --git a/xen/drivers/char/ns16550.c b/xen/drivers/char/ns16550.c
index c174465059..278d38822d 100644
--- a/xen/drivers/char/ns16550.c
+++ b/xen/drivers/char/ns16550.c
@@ -427,6 +427,10 @@ static void __init cf_check ns16550_init_preirq(struct
serial_port *port)
#endif
}
+ /* Preserve output queued by firmware before taking over the UART. */
+ while ( !(ns_read_reg(uart, UART_LSR) & UART_LSR_TEMT) )
+ cpu_relax();
+
ns16550_setup_preirq(uart);
/* Check this really is a 16550+. Otherwise we have no FIFOs. */
|
