Re: [PATCH net-next v2 1/2] net: ethernet: mtk_eth_soc: use prefetch methods

Stefan Roese
2024-08-01T07:19:41Z

On 7/30/24 20:35, Elad Yifee wrote:
> On Tue, Jul 30, 2024 at 11:59 AM Joe Damato <jdamato@fastly.com> wrote:
>>
>> Based on the code in mtk_probe, I am guessing that only
>> MTK_SOC_MT7628 can DMA to unaligned addresses, because for
>> everything else eth->ip_align would be 0.
>>
>> Is that right?
>>
>> I am asking because the documentation in
>> Documentation/core-api/unaligned-memory-access.rst refers to the
>> case you mention, NET_IP_ALIGN = 0, suggesting that this is
>> intentional for performance reasons on powerpc:
>>
>> One notable exception here is powerpc which defines NET_IP_ALIGN to
>> 0 because DMA to unaligned addresses can be very expensive and dwarf
>> the cost of unaligned loads.
>>
>> It goes on to explain that some devices cannot DMA to unaligned
>> addresses and I assume that for your driver that is everything which
>> is not MTK_SOC_MT7628 ?
>
> I have no explanation for this partial use of 'eth->ip_align', it
> could be a mistake
> or maybe I'm missing something.
> Perhaps Stefan Roese, who wrote this part, has an explanation.
> (adding Stefan to CC)

Sorry, I can't answer this w/o digging deeper into this driver and
SoC again. And I didn't use it for a few years now. It might be a
mistake.

Thanks,
Stefan