Re: [PATCH net-next] tg3: normalize inherited M3000 register byte order

[Magnus Lindholm]

Hi Andrew,

On Fri, Oct 9, 2026 at 2:09 PM Andrew Lunn <andrew@xxxxxxx> wrote:
>
> > > Is there a legitimate reason you would want the bytes are swapped?
>
> You missed answering this. I'm guessing, but does the hardware offer
> this to make big endian machines more efficient?
>
> Please could you look at the driver and think about what would happen
> on a big endian machine. Is the change still correct?
>
>         Andrew

You're right, my earlier reply missed the underlying question.

That is a plausible reason for providing the hardware swap mode:
it can accommodate big-endian accesses without software byte
swapping. I haven't verified Broadcom's design rationale.

However, Linux tg3 already initializes misc_host_ctrl with WORD_SWAP set
and BYTE_SWAP clear on both little- and big-endian builds, before
this helper runs.

The helper restores that existing setting. Its swab32() compensates
for the device's inherited register state, not CPU endianness.
Subsequent accesses use the normal driver path, and the separate
__BIG_ENDIAN handling for DMA remains unchanged.

That is why I believe the correction is appropriate on either
endianness. The runtime tests were on the M3000 running big-endian
SPARC64.

Regards,
Magnus