linux-kernel - Re: [PATCH v2 02/16] thunderbolt: Don't pass a bitfield to FIELD

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Message-ID: <aTzPT2kAt96ypGU-@yury>
Date: Fri, 12 Dec 2025 21:28:31 -0500
From: Yury Norov <yury.norov@...il.com>
To: david.laight.linux@...il.com
Cc: Rasmus Villemoes <linux@...musvillemoes.dk>,
	linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org,
	Geert Uytterhoeven <geert+renesas@...der.be>,
	Alexandre Belloni <alexandre.belloni@...tlin.com>,
	Jonathan Cameron <Jonathan.Cameron@...wei.com>,
	Crt Mori <cmo@...exis.com>,
	Richard Genoud <richard.genoud@...tlin.com>,
	Andy Shevchenko <andriy.shevchenko@...el.com>,
	Luo Jie <quic_luoj@...cinc.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
	"David S . Miller" <davem@...emloft.net>,
	Mika Westerberg <mika.westerberg@...ux.intel.com>,
	Andreas Noever <andreas.noever@...il.com>,
	Yehezkel Bernat <YehezkelShB@...il.com>,
	Nicolas Frattaroli <nicolas.frattaroli@...labora.com>
Subject: Re: [PATCH v2 02/16] thunderbolt: Don't pass a bitfield to FIELD_GET

On Fri, Dec 12, 2025 at 07:37:07PM +0000, david.laight.linux@...il.com wrote:
> From: David Laight <david.laight.linux@...il.com>
> 
> None of sizeof(), typeof() or __auto_type can be used with bitfields
> which makes it difficult to assign a #define parameter to a local
> without promoting char and short to int.
> 
> Change:
> 	u32 thunderbolt_version:8;
> to the equivalent:
> 	u8 thunderbolt_version;
> (and the other three bytes of 'DWORD 4' to match).
> 
> This is necessary so that FIELD_GET can use sizeof() to verify 'reg'.
> 
> Signed-off-by: David Laight <david.laight.linux@...il.com>
> ---
> 
> Changes for v2:
> - Change structure definition instead of call to FIELD_GET().
> 
> FIELD_GET currently uses _Generic() which behaves differently for
> gcc and clang (I suspect both are wrong!).
> gcc treats 'u32 foo:8' as 'u8', but will take the 'default' for other
> widths (which will generate an error in FIED_GET().
> clang treats 'u32 foo:n' as 'u32'.

FIELD_GET() works just well with bitfields, and whatever you do breaks
it. I pointed that in v1, but instead of fixing it, you do really well
hiding the problem.

I see no reasons to hack a random victim because of your rework. So
NAK for this. 

In v3, please add an explicit test to make sure that bitfields are not
broken with new implementation.

Thanks,
Yury