Re: [PATCH 6/6] lib/scatterlist: Drop order argument from sgl_free_n_order

[Tvrtko Ursulin]

Hi,

On 07/03/18 18:30, James Bottomley wrote:
> On Wed, 2018-03-07 at 12:47 +0000, Tvrtko Ursulin wrote:
> > From: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
>
> Firstly, I don't see any justifiable benefit to churning this API, so
> why bother? but secondly this:

Primarily because I wanted to extend sgl_alloc_order slightly in order 
to be able to use it from i915. And then in the process noticed a couple 
of bugs in the implementation, type inconsistencies and unused exported 
symbols. That gave me a feeling API could actually use a bit of work.

> > We can derive the order from sg->length and so do not need to pass it
> > in explicitly.
>
> Is wrong. ÂI can have a length 2 scatterlist that crosses a page
> boundary, but I can also have one within a single page, so the order
> cannot be deduced from the length.

sgl_alloc_order never does this.

However there is a different bug in my patch relating to the last entry 
which can have shorter length from the rest. So get_order on the last 
entry is incorrect - I have to store the deduced order and carry it over.

In which case it may even make sense to refactor sgl_alloc_order a bit 
more to avoid wastage on the last entry with high order allocations.

Regards,

Tvrtko