Re: [PATCH] alpha/PCI: Fix I/O port accessor argument order in pci_legacy_write()

[Magnus Lindholm]

On Sat, Jun 13, 2026 at 1:24 AM Krzysztof Wilczyński
<kwilczynski@xxxxxxxxxx> wrote:
>
> pci_legacy_write() in arch/alpha/kernel/pci-sysfs.c passes its arguments
> to outb(), outw() and outl() in the wrong order:
>
>   outb(port, val);
>
> The Alpha I/O accessors in arch/alpha/include/asm/io.h take the value
> first and the port second:
>
>   extern void outb(u8 b, unsigned long port);
>
> So the port number is written as data to the I/O address taken from the
> user-supplied value, and the intended write to the requested port never
> happens.
>
> The arguments have been reversed since the file was added, and the
> function returns the access size regardless, so the caller sees success
> while the requested port is left untouched.
>
> Fixes: 10a0ef39fbd1 ("PCI/alpha: pci sysfs resources")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Krzysztof Wilczyński <kwilczynski@xxxxxxxxxx>
> ---
>  arch/alpha/kernel/pci-sysfs.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/arch/alpha/kernel/pci-sysfs.c b/arch/alpha/kernel/pci-sysfs.c
> index 3048758304b5..354ec5f10ad0 100644
> --- a/arch/alpha/kernel/pci-sysfs.c
> +++ b/arch/alpha/kernel/pci-sysfs.c
> @@ -355,17 +355,17 @@ int pci_legacy_write(struct pci_bus *bus, loff_t port, u32 val, size_t size)
>
>         switch(size) {
>         case 1:
> -               outb(port, val);
> +               outb(val, port);
>                 return 1;
>         case 2:
>                 if (port & 1)
>                         return -EINVAL;
> -               outw(port, val);
> +               outw(val, port);
>                 return 2;
>         case 4:
>                 if (port & 3)
>                         return -EINVAL;
> -               outl(port, val);
> +               outl(val, port);
>                 return 4;
>         }
>         return -EINVAL;
> --

Krzysztof,

Well spotted, this looks correct to me.

This must have been lingering here unnoticed for quite some time.  The
Alpha outb/outw/outl accessors take the value first and the port second,
so the current pci_legacy_write() argument order is clearly wrong.

I tested this on an AlphaStation DS10 by exercising the PCI legacy_io
sysfs path against the scratch registers of the onboard 16550A UARTs at
0x3f8/0x2f8.  With the fix applied, writes through legacy_io read back
correctly:

  ttyS0 scratch at 0x3ff: 0x5a/0xa5 write/readback passed
  ttyS1 scratch at 0x2ff: 0x5a/0xa5 write/readback passed

For comparison, attempting the same test on the unpatched kernel caused
the machine to hang, which is consistent with the reversed arguments
turning the intended UART scratch-register write into an access to an
unrelated legacy I/O port.

I will pick this up.  It might not make this merge window, but I will try
to get it queued sooner rather than later.

Reviewed-by: Magnus Lindholm <linmag7@xxxxxxxxx>
Tested-by: Magnus Lindholm <linmag7@xxxxxxxxx>