[U-Boot] [PATCH] cmd: net: flush cache cacheline aligned

[Joe Hershberger]

Hi Simon,

On Wed, Aug 3, 2016 at 8:17 PM, Simon Glass <sjg at chromium.org> wrote:
> Hi Joe,
>
> On 3 August 2016 at 17:18, Joe Hershberger <joe.hershberger at gmail.com> wrote:
>> On Tue, Aug 2, 2016 at 2:20 AM, Stefan Agner <stefan at agner.ch> wrote:
>>> From: Stefan Agner <stefan.agner at toradex.com>
>>>
>>> Flush loaded data cacheline aligned. This avoids warnings such as
>>> CACHE: Misaligned operation at range [81000000, 816d0fa8]
>>>
>>> Signed-off-by: Stefan Agner <stefan.agner at toradex.com>
>>> ---
>>
>> This was already rejected once.
>> http://lists.denx.de/pipermail/u-boot/2012-April/121564.html
>>
>>> Why do we actually have to flush caches after load? It seems to
>>> have worked so far despite the caches did not get flushed (due to
>>> missalignment).
>>
>> I'm not sure that we do, but it's been there since as far back as the
>> git history goes. Maybe Wolfgang has memory of a reason.
>
> I think this is the correct solution. If you load something to memory
> you have to flush the cache. If someone is brave enough to load an
> image that finishes immediately before something where there is data
> that will be overwritten by the flush, then they deserve some pain.

I'm not at all worried about the overwrite case.

> Anytime we flush the cache we have to do it according the rules of the
> cache. One of those rules is that cache lines are larger than one
> byte. So either we don't flush or we do, and if we do, we must align.

For sure... I was just wondering why we need this flush at all.
Stefan's comment as to why seems reasonable now, but that reasoning
surely wasn't a concern pre-2000. Maybe some early arch needed it for
a different reason? Maybe someone was planning to DMA directly from
the memory that was loaded, but in that case it seems like the code
starting the DMA would ensure the flush, not the code that is
memcpy'ing from somewhere.

Cheers,
-Joe