linux-kernel - Re: Sparse warning when using ioread64() from include/asm-generic/io.h

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Message-ID: <20230109114016.z6qowgppuh7uoj75@skbuf>
Date:   Mon, 9 Jan 2023 13:40:16 +0200
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Linux-Arch <linux-arch@...r.kernel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: Sparse warning when using ioread64() from
 include/asm-generic/io.h

On Mon, Jan 09, 2023 at 12:33:31PM +0100, Arnd Bergmann wrote:
> On Mon, Jan 9, 2023, at 11:40, Vladimir Oltean wrote:
> > Hi,
> >
> > I would like to get rid of the following sparse error in the enetc
> > driver (for arm64), which uses ioread64().
> >
> > ../drivers/net/ethernet/freescale/enetc/enetc_ethtool.c: note: in included file
> > 	(through ../arch/arm64/include/asm/io.h, ../include/linux/io.h,
> > 	../include/linux/irq.h, ../include/asm-generic/hardirq.h,
> > 	../arch/arm64/include/asm/hardirq.h, ...):
> > ../include/asm-generic/io.h:239:15: warning: cast to restricted __le64
> >
> > The trouble is I don't understand why the casts to __le64 and use of
> > __le64_to_cpu() are even needed, when everything seems to be native
> > endianness. I've seen commit c1d55d50139b ("asm-generic/io.h: Fix sparse
> > warnings on big-endian architectures"), but that doesn't claim to fix
> > anything for little endian (and doesn't touch the 64 accessors, for some
> > reason).
> >
> > Could you please help?
> 
> From what I can tell, the fix for openrisc was described as
> a big-endian warning fix, but the warning is actually the same
> on both. The difference is that on little-endian kernels,
> the __le64_to_cpu() conversion only changes the type but not
> the value, while on big-endian machines, the value would
> be wrong without the conversion: __raw_readl() is defined
> to never byteswap the data, while readl() must byteswap
> little-endian MMIO registers into big-endian CPU registers
> when CONFIG_CPU_BIG_ENDIAN is set.
> 
> Since Stafford only tested on 32-bit OpenRISC, he missed the
> readq()/writeq() accessors that need the same warning fix.
> 
>      Arnd

Hmm, ok, didn't know that difference between __raw_readq() and readq().
With this information, I guess I can submit a patch. Thanks!