Re: [PATCH] of: Allow mem_reserve of memory with a base address of zero

[Laura Abbott <lauraa-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org>]

On 8/12/2014 9:48 AM, Grant Likely wrote:
> On Wed, 6 Aug 2014 17:02:44 -0500, Rob Herring <robherring2-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>> On Wed, Aug 6, 2014 at 3:30 PM, Al Cooper <alcooperx-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
>>> __reserved_mem_reserve_reg() won't reserve memory if the base address
>>> is zero. This change removes the check for a base address of zero and
>>> allows it to be reserved.
>>>
>>> Allowing the first 4K of memory to be reserved will help solve a
>>> problem on some ARM systems where the the first 16K of memory is
>>> unused and becomes allocable memory. This will prevent this memory
>>> from being used for DMA by drivers like the USB OHCI driver which
>>> consider a physical address of zero to be illegal.
>>
>> OHCI driver or hardware? I agree with the change, but really think
>> this should be fixed in the driver in the former case or a property of
>> the OHCI node in the latter.
> 
> I'm not hard and fast on that. I don't think it is unreasonable to
> reserve the base of memory as the platform level if there is a bug
> causing DMA to fail at that address.
> 
> If this is a common problem (and not merely a few boards) then I agree.
> Fixing it this way is just asking for the same problem to show up again
> and again on new boards that haven't explicitly reserved the memory.
> 
> g.
> 

Reserving memory at physical address 0 has been semi-common on our boards
for many issues. Sometimes it's been for temporary debugging purposes
to catch an unknown driver corrupting physical address 0. Sometimes it has
been a permanent fix to a hardware limitation.

Thanks,
Laura

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