Re: [PATCH v2 2/3] xen/uart: be more careful with changes to the PCI command register

["Roger Pau Monné" <roger.pau@citrix.com>]

On Mon, Mar 30, 2026 at 10:00:05AM +0200, Jan Beulich wrote:
> On 27.03.2026 14:54, 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, which might put the device into a non-working state.
> > 
> > Fixes: f2ff5d6628b3 ("ns16550: enable PCI serial card usage")
> > Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
> 
> Reviewed-by: Jan Beulich <jbeulich@suse.com>
> 
> I would have preferred if the description mentioned the particular case,
> turning this more into a workaround than an apparent bugfix. 

It turns out that the console does seem to work fine, even with memory
decoding disabled on the device (as expected).  I've updated the
firmware in the meantime, so I'm unsure whether that update has
changed the behavior of the device, or it simply was some other
instability that was causing the issue in the past.  This SOL AMT
device is not reliable at all I'm afraid.

> As mentioned,
> us driving the device generally means we're free to do whatever we want to
> the command register, as long as resulting device state is consistent
> overall (or else we may indeed have a non-working device). Having to keep
> memory decoding enabled in order for I/O ports to function is pretty
> clearly a bug in the device, and hence us "violating" that requirement
> isn't really o bug of ours.

I think given the fragility of some of those SOL devices it's best to
limit the number of bits Xen changes, as to having a bigger chances of
getting output working.

Thanks, Roger.