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Re: [PATCH v3] dp8393x: don't force 32-bit register access

From: Philippe Mathieu-Daudé
Subject: Re: [PATCH v3] dp8393x: don't force 32-bit register access
Date: Mon, 5 Jul 2021 23:36:23 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0

On 7/5/21 9:33 PM, Mark Cave-Ayland wrote:
> On 05/07/2021 08:52, Mark Cave-Ayland wrote:
>> I think the problem is because of the interaction of
>> .impl.max_access_size = 2 and the it_shift property specifying a
>> stride of 4 bytes: when the 4 byte access is split into 2 x 2 byte
>> accesses then for a read reg = addr >> s->it_shift causes the second
>> access to be a duplicate of the first rather than containing zeros.

I believe this is something I tried to fix earlier, see:

"hw/misc: Add support for interleaved memory accesses"
(in particular:


"hw/misc: Add memory_region_add_subregion_aliased() helper"

I plan to respin during next dev cycle...

> After some more experiments I'm fairly confident this is the issue: if
> you rely on applying it_shift within the MMIO access itself then you
> lose the zero extension of the result to the access size as done by the
> memory API.
> I'll come up with a new version which I shall send as part of an updated
> and tested v2 of Phil's housekeeping patchset, since the endian changes
> were really helpful when studying the descriptors.
I'm fine if we take the patch Finn has tested (I'll amend a comment
explaining the limitations) and we fix the details in the next cycle.

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