netdev - Re: [PATCH net-next v3 1/4] net: dsa: yt921x: Use *_ULL bitfield macros for VLAN

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Message-ID: <aSmK1T4maiYysTZ0@shell.armlinux.org.uk>
Date: Fri, 28 Nov 2025 11:43:17 +0000
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: david laight <david.laight@...box.com>
Cc: David Yang <mmyangfl@...il.com>, netdev@...r.kernel.org,
	Andrew Lunn <andrew@...n.ch>, Vladimir Oltean <olteanv@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Simon Horman <horms@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v3 1/4] net: dsa: yt921x: Use *_ULL bitfield
 macros for VLAN_CTRL

On Fri, Nov 28, 2025 at 10:51:41AM +0000, david laight wrote:
> On Wed, 26 Nov 2025 17:32:34 +0800
> David Yang <mmyangfl@...il.com> wrote:
> 
> > VLAN_CTRL should be treated as a 64-bit register. GENMASK and BIT
> > macros use unsigned long as the underlying type, which will result in a
> > build error on architectures where sizeof(long) == 4.
> 
> I suspect GENMASK() should generate u32 or u64 depending on the value
> of a constant 'high bit'.

I suggest checking before making such statements to save embarrasment.
The above is incorrect.

#define GENMASK_TYPE(t, h, l)                                   \
        ((t)(GENMASK_INPUT_CHECK(h, l) +                        \
             (type_max(t) << (l) &                              \
              type_max(t) >> (BITS_PER_TYPE(t) - 1 - (h)))))

#define GENMASK(h, l)           GENMASK_TYPE(unsigned long, h, l)
#define GENMASK_ULL(h, l)       GENMASK_TYPE(unsigned long long, h, l)

#define GENMASK_U8(h, l)        GENMASK_TYPE(u8, h, l)
#define GENMASK_U16(h, l)       GENMASK_TYPE(u16, h, l)
#define GENMASK_U32(h, l)       GENMASK_TYPE(u32, h, l)
#define GENMASK_U64(h, l)       GENMASK_TYPE(u64, h, l)
#define GENMASK_U128(h, l)      GENMASK_TYPE(u128, h, l)

Note that GENMASK(33, 15) will fail to build on 32-bit systems.

> I found code elsewhere that doesn't really want FIELD_PREP() to
> generate a 64bit value.
> 
> There are actually a lot of dubious uses of 'long' throughout
> the kernel that break on 32bit.
> (Actually pretty much all of them!)

If you're referring to the use of GENMASK() with bitfields larger
than 32-bits, then as can be seen from the above, the code wouldn't
even compile and our CI systems would be screaming about it. They
aren't, so I think your statement here is also wrong.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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