RE: [PATCH V7 2/4] asm-generic/io.h: Add big-endian MMIO accessors

[Guntupalli, Manikanta]

[Public]

Hi,

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> Subject: Re: [PATCH V7 2/4] asm-generic/io.h: Add big-endian MMIO accessors
>
> On Tue, Sep 23, 2025, at 17:45, Manikanta Guntupalli wrote:
> > Add MMIO accessors to support big-endian memory operations. These
> > helpers include {read, write}{w, l, q}_be() and {read, write}s{w, l,
> > q}_be(), which allows to access big-endian memory regions while
> > returning the results in the CPU’s native endianness.
> >
> > This provides a consistent interface to interact with hardware using
> > big-endian register layouts.
> >
> > Signed-off-by: Manikanta Guntupalli <manikanta.guntupalli@xxxxxxx>
>
> I feel like we already have too many accessor functions like these, what's wrong with
> just using io{read,write}{8,16,32,64}be() in your driver?
>
> On most architectures (including arm, riscv, powerpc and microblaze, but not x86),
> the ioread/write helpers are identical to the readl/writel style helpers, the only
> difference being that on x86 they add an extra indirection for the port I/O check.
>
> At the moment, there are only six drivers that use the
> io{read,write}{8,16,32,64}be() style helpers. They are all powerpc specific and can
> probably be changed to io{read,write}be.

The existing helpers io{read,write}{8,16,32,64}be() internally rely on {read,write}{b,w,l,q}().

From both description and implementation, {read,write}{b,w,l,q}() access little-endian memory and return the result in native endianness:

/*
 * {read,write}{b,w,l,q}() access little endian memory
 * and return result in native endianness.
 */
static inline u32 readl(const volatile void __iomem *addr)
{
        u32 val;

        log_read_mmio(32, addr, _THIS_IP_, _RET_IP_);
        __io_br();
        val = __le32_to_cpu((__le32 __force)__raw_readl(addr));
        __io_ar(val);
        log_post_read_mmio(val, 32, addr, _THIS_IP_, _RET_IP_);
        return val;
}

The io{read,write}*be() helpers then perform a byte swap, e.g.:

static inline u32 ioread32be(const volatile void __iomem *addr)
{
        return swab32(readl(addr));
}

This works on little-endian CPUs, but on big-endian CPUs the {read,write}{b,w,l,q}() helpers already return data in big-endian format. The extra byte swap in io{read,write}*be() therefore corrupts the data, producing little-endian values when big-endian is expected.

The newly introduced {read,write}{w,l,q}_be() helpers directly access big-endian IO memory and return results in native endianness, avoiding this mismatch.


Thanks,
Manikanta.