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Re: [PATCH 1/4] xen/uart: be more careful with changes to the PCI command register
- To: Roger Pau Monne <roger.pau@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Thu, 26 Mar 2026 11:37:57 +0000
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>
- Delivery-date: Thu, 26 Mar 2026 11:38:18 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 25/03/2026 2:58 pm, Roger Pau Monne wrote:
> Read the existing PCI command register and only add the required bits to
> it, as to avoid clearing bits that might be possibly set by the firmware
> already.
>
> This fixes serial output when booting with `com1=device=amt` on a system
> using an "Alder Lake AMT SOL Redirection" PCI device (Vendor ID 0x8086 and
> Device ID 0x51e3). That device has both IO and memory decoding enabled by
> the firmware, and disabling memory decoding causes the serial to stop
> working (even when the serial register BAR is in the IO space).
>
> Fixes: f2ff5d6628b3 ("ns16550: enable PCI serial card usage")
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
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