[PATCH 3/5] drm/panfrost: Add a no execute flag for BO allocations

[Steven Price]

On 22/07/2019 17:18, Rob Herring wrote:
> On Mon, Jul 22, 2019 at 7:25 AM Robin Murphy <robin.murphy at arm.com> wrote:
>>
>> On 22/07/2019 13:19, Steven Price wrote:
>> [...]
>>> Indeed, that case was just occurring to me too! How about:
>>>
>>>       u64 next_seg = ALIGN(*start, PFN_4G);
>>>
>>>       if (next_seg - *start <= PFN_16M)
>>>               *start = next_seg + 1;
> 
> This could make start > end...

But only in the case where (*end - *start < 16MB) - i.e. this region
wasn't big enough. The generic code will deal with that and reject the hole.

> It also doesn't handle not starting on a 4G boundary (or was that
> condition check supposed to be included still).

I was assuming we still had this before that code:

	if ((*start & PFN_4G_MASK) == 0)
		(*start)++;

	if ((*end & PFN_4G_MASK) == 0)
		(*end)--;

And when bumping *start to next_seg, there's a +1 to deal with the 4GB
boundary case.

Steve

>>>
>>>       *end = min(*end, ALIGN(*start, PFN_4G) - 1);
>>>
>>> So always allocate at the beginning, but skip past the next 4GB boundary
>>> if there's less than 16MB left (or equal to avoid the 4GB boundary).
>>
>> Ah, there it is! I think it generalises even further by just changing
>> the condition to "if (next_seg - *start < *end - next_seg)", although
>> we'd need to ensure a signed comparison to cover the case where start
>> and end are already in the same segment.
> 
> IMO, relying on signed comparsion doesn't really improve the readability...
> 
> Rob
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