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Re: [PATCH v2 2/2] xen/ns16550: drain TX before reinitializing
- To: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 12 Oct 2026 15:00:19 +0200
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- Cc: Alistair Francis <alistair.francis@xxxxxxx>, Connor Davis <connojdavis@xxxxxxxxx>, Baptiste Le Duc <baptiste.le-duc@xxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Julien Grall <julien@xxxxxxx>, Roger Pau Monné <roger@xxxxxxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Zhang Zheng <zhangzheng@xxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Mon, 12 Oct 2026 13:00:22 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 12.10.2026 12:20, Oleksii Kurochko wrote:
> On 10/10/26 5:14 AM, Zhang Zheng wrote:
>> --- 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. */
> я'
> Could the comment say why the wait is needed at this point? Something
> like:
>
> /*
> * Output written before ns16550 takes over (e.g. by firmware or the
> * early console) may still be in the transmitter. Let it drain before
> * ns16550_setup_preirq() reprograms the line settings and resets the
> * FIFOs.
> */
>
>> + while ( !(ns_read_reg(uart, UART_LSR) & UART_LSR_TEMT) )
>> + cpu_relax();
>> +
>
> IMO waiting for TEMT rather than THRE is the right choice: besides
> resetting the FIFOs, ns16550_setup_preirq() reprograms the divisor and
> LCR, which would corrupt a character still being shifted out. (Linux
> waits for UART_LSR_BOTH_EMPTY, i.e. THRE | TEMT, but on a 16550 TEMT
> already implies THRE.)
Then again we have the .lsr_mask field. Not using it here would then call
for justification in the description.
Jan
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