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Re: [PATCH 1/4] dp8393x: don't force 32-bit register access

From: Finn Thain
Subject: Re: [PATCH 1/4] dp8393x: don't force 32-bit register access
Date: Thu, 8 Jul 2021 10:52:22 +1000 (AEST)

On Wed, 7 Jul 2021, Mark Cave-Ayland wrote:

> However this conflicts with what you mention above that the SONIC is 
> hard-coded into little-endian mode, in which case we would still need to 
> keep it.

If you want to fully implement BMODE in QEMU then you'll need to abandon 
native endiannes for the device implementation. I was not proposing this 
as it implies more byte swapping.

In a real Magnum the SONIC chip is connected to a bus that's not modelled 
by QEMU. It follows that BMODE serves different purposes than big_endian. 

I pointed out several semantic differences between BMODE and big_endian, 
but I think the most significant of those was that endianness is already a 
property of the memory device being accessed for DMA. Yet big_endian is a 
property of the dp8393x device.

> Certainly we can look to improve things in the future, but without 
> anyone having a working big-endian MIPS image to test against, I don't 
> think it's worth guessing what changes are required as we can easily 
> double the length of this thread and still have no idea if any changes 
> we've made are correct.

That argument can be applied to other patches in this series also.

Anyway, if we agree that the aim is ultimately to remove the big_endian 
flag then patch 4/4 should probably be re-evaluated in light of that.

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