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

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

[Vladimir Oltean]

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?

Thanks,
Vladimir

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

[Arnd Bergmann]

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

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

[Vladimir Oltean]

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!