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Re: [PATCH v2 1/2] xen/ns16550: detect and preserve UART_IER_UUE
- To: Zhang Zheng <zhangzheng@xxxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 12 Oct 2026 14:52:06 +0200
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- Cc: Alistair Francis <alistair.francis@xxxxxxx>, Connor Davis <connojdavis@xxxxxxxxx>, Oleksii Kurochko <oleksii.kurochko@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>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Mon, 12 Oct 2026 12:52:09 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 10.10.2026 05:14, Zhang Zheng wrote:
> --- a/xen/drivers/char/ns16550.c
> +++ b/xen/drivers/char/ns16550.c
> @@ -64,6 +64,7 @@ static struct ns16550 {
> bool intr_works;
> bool force_polling;
> bool dw_usr_bsy;
> + bool xscale;
> #ifdef NS16550_PCI
> /* PCI card parameters. */
> bool pb_bdf_enable; /* if =1, pb-bdf effective, port behind bridge */
> @@ -352,6 +353,14 @@ static void ns16550_setup_preirq(struct ns16550 *uart)
> /* No interrupts. */
> ns_write_reg(uart, UART_IER, 0);
>
> + uart->xscale = false;
> + if ( !(ns_read_reg(uart, UART_IER) & UART_IER_UUE) )
> + {
> + ns_write_reg(uart, UART_IER, UART_IER_UUE);
> + if ( ns_read_reg(uart, UART_IER) & UART_IER_UUE )
> + uart->xscale = true;
> + }
Linux'es detection is a little more involved, plus: It doesn't clobber the
value originally present in the other 7 bits.
Jan
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