Re: [PATCH v4] hwmon: (dell-smm) Improve assembly code

[Guenter Roeck <linux@xxxxxxxxxxxx>]

On 2/23/22 14:35, David Laight wrote:
> From: Armin Wolf
> > Sent: 23 February 2022 21:39
> ...
> > > As far as this patch goes I think I'd add a 'stc' (set carry)
> > > instruction before the first 'outb'.
> > > Then if the 'something magic happens here' doesn't happen (because
> > > you aren't on the right kind of motherboard) the action fails.
> > >
> > > 	David
> >
> > We already check for such scenarios by checking if eax changed.
> > I would not set the carry flag before doing a SMM call since im
> > not sure if the firmware code always clears the carry flag when
> > the call was successful.
> > If for example the firmware code only sets the carry flag on
> > error and otherwise ignores the carry flag, we might create some
> > false negatives when a successful SMM call leaves the carry flag set.
>
> If you are worried about that you should be clearing carry on entry.

I agree, but now we have an argument for clearing carry (because the
SMM code might not do it) and for setting carry (because the SMM code
might not execute). I don't know which way to go, if any, but that
would be a functional change and should be submitted as separate patch.

Guenter