Subject: Re: [RFC] genalloc:add an gen_pool_alloc_align func to genalloc

From: Scott Wood <[email-masked]>
Date: Thu, 9 Jul 2015 17:17:02 -0500

On Thu, 2015-07-09 at 14:51 -0700, Laura Abbott wrote:
> On 07/09/2015 12:47 AM, Zhao Qiang wrote:
> > Bytes alignment is required to manage some special ram,
> > so add gen_pool_alloc_align func to genalloc.
> > rename gen_pool_alloc to gen_pool_alloc_align with a align parameter,
> > then provide gen_pool_alloc to call gen_pool_alloc_align with
> > align = 1 Byte.
> > 
> > Signed-off-by: Zhao Qiang <[email-masked]>
> > ---
> > FSL's IP block QE require this function to manage muram.
> > QE supported only PowerPC, and its code was put under arch/powerpc 
> > directory,
> > using arch/powerpc/lib/rheap.c to manage muram.
> > Now it support both arm(ls1021,ls1043,ls2085 and such on) and powerpc,
> > the code need to move from arch/powerpc to public direcory,
> > Scott wood hopes to use genalloc to manage the muram, after discussing
> > with scott, we decide to add gen_pool_alloc_align to meet the requirement
> > for bytes-alignment.
> 
> gen_pool supports custom allocation algorithms. I thought this was discussed
> previously and the conclusion was that if you wanted alignment you should
> use custom allocation algorithms. I'm failing at finding any thread 
> discussing it though.

I hope that by "custom algorithm" you don't mean something implemented 
outside lib/genalloc.c, as this does not seem like such a specialized 
requirement that everyone must reimplement it separately.

> Perhaps another option would be to add another runtime argument to gen_pool
> where you could pass the alignment to your custom allocation function. This
> way alignment isn't inherently coded into any of the algorithms.

That wouldn't let the alignment change for each allocation (and could already 
be done with pool->data).  I suppose one could call get_pool_set_algo() with 
different data (or modify the memory that pool->data is already pointing to) 
before each allocation, but that's a bit clunky...  If making alignment part 
of the mainstream flow is undesired for some reason, how about a 
gen_pool_alloc_data() that lets it be passed in per-allocation (with 
gen_pool_alloc() being a wrapper that passes in pool->data)?

Yes, I know, we could do it in a wrapper (like cpm_muram_alloc() 
unnecessarily does), but why not make the interface better match the way it's 
used?

-Scott