linux-kernel - Re: [PATCH 2/9] thunderblot: Don't pass a bitfield to FIELD

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Message-ID: <20251209103621.7e6922ab@pumpkin>
Date: Tue, 9 Dec 2025 10:36:21 +0000
From: David Laight <david.laight.linux@...il.com>
To: Greg KH <gregkh@...uxfoundation.org>
Cc: Yury Norov <yury.norov@...il.com>, Rasmus Villemoes
 <linux@...musvillemoes.dk>, linux-kernel@...r.kernel.org,
 linux-usb@...r.kernel.org, Geert Uytterhoeven <geert+renesas@...der.be>
Subject: Re: [PATCH 2/9] thunderblot: Don't pass a bitfield to FIELD_GET

On Tue, 9 Dec 2025 07:56:35 +0900
Greg KH <gregkh@...uxfoundation.org> wrote:

> On Mon, Dec 08, 2025 at 10:42:43PM +0000, david.laight.linux@...il.com wrote:
> > From: David Laight <david.laight.linux@...il.com>
> > 
> > FIELD_GET needs to use __auto_type to get the value of the 'reg'
> > parameter, this can't be used with bifields.
> > 
> > FIELD_GET also want to verify the size of 'reg' so can't add zero
> > to force the type to int.
> > 
> > So add a zero here.
> > 
> > Signed-off-by: David Laight <david.laight.linux@...il.com>
> > ---
> >  drivers/thunderbolt/tb.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/thunderbolt/tb.h b/drivers/thunderbolt/tb.h
> > index e96474f17067..7ca2b5a0f01e 100644
> > --- a/drivers/thunderbolt/tb.h
> > +++ b/drivers/thunderbolt/tb.h
> > @@ -1307,7 +1307,7 @@ static inline struct tb_retimer *tb_to_retimer(struct device *dev)
> >   */
> >  static inline unsigned int usb4_switch_version(const struct tb_switch *sw)
> >  {
> > -	return FIELD_GET(USB4_VERSION_MAJOR_MASK, sw->config.thunderbolt_version);
> > +	return FIELD_GET(USB4_VERSION_MAJOR_MASK, sw->config.thunderbolt_version + 0);  
> 
> This is going to be very confusing to people who see this line only.
> Can we add a comment here to explain why we have to do a "+ 0" and why
> it can't be removed?  Otherwise I'm going to get a bunch of "cleanup"
> patches attempting to "fix" this over the next few years.

I've added a comment for v2 (but not the resend I did for the cc 'issue').
I've also fixed the subject blot => bolt.

I also noticed that the 'int thunderbolt_version:8' seems to be in a structure
that maps something read from the hardware.
That isn't going to be right on any BE system, and isn't guaranteed on LE ones.

	David

> 
> thanks,
> 
> greg k-h