Re: [PATCH v6 1/3] genalloc:support memory-allocation with bytes-alignment to genalloc

[Scott Wood <scottwood@freescale.com>]

On Tue, 2015-08-25 at 03:09 -0500, Zhao Qiang-B45475 wrote:
> On 08/25/2015 12:01 PM, Laura Abbott wrote:
> 
> > -----Original Message-----
> > From: Laura Abbott [mailto:labbott@redhat.com]
> > Sent: Tuesday, August 25, 2015 12:01 PM
> > To: Zhao Qiang-B45475; Wood Scott-B07421
> > Cc: linux-kernel@vger.kernel.org; linuxppc-dev@lists.ozlabs.org;
> > lauraa@codeaurora.org; Xie Xiaobo-R63061; benh@kernel.crashing.org; Li
> > Yang-Leo-R58472; paulus@samba.org
> > Subject: Re: [PATCH v6 1/3] genalloc:support memory-allocation with
> > bytes-alignment to genalloc
> > 
> > On 08/24/2015 07:40 PM, Zhao Qiang wrote:
> > > On 08/25/2015 07:11 AM, Laura Abbott wrote:
> > > > -----Original Message-----
> > > > From: Laura Abbott [mailto:labbott@redhat.com]
> > > > Sent: Tuesday, August 25, 2015 7:11 AM
> > > > To: Zhao Qiang-B45475; Wood Scott-B07421
> > > > Cc: linux-kernel@vger.kernel.org; linuxppc-dev@lists.ozlabs.org;
> > > > lauraa@codeaurora.org; Xie Xiaobo-R63061; benh@kernel.crashing.org;
> > > > Li Yang-Leo-R58472; paulus@samba.org
> > > > Subject: Re: [PATCH v6 1/3] genalloc:support memory-allocation with
> > > > bytes-alignment to genalloc
> > > > 
> > > > On 08/24/2015 02:31 AM, Zhao Qiang wrote:
> > > > > Bytes alignment is required to manage some special RAM, so add
> > > > > gen_pool_first_fit_align to genalloc, meanwhile add
> > > > > gen_pool_alloc_data to pass data to gen_pool_first_fit_align(modify
> > > > > gen_pool_alloc as a wrapper)
> > > > > 
> > > > > Signed-off-by: Zhao Qiang <qiang.zhao@freescale.com>
> > > > > ---
> > > > > Changes for v6:
> > > > >       - patches set v6 include a new patch because of using
> > > > >       - genalloc to manage QE MURAM, patch 0001 is the new
> > > > >       - patch, adding bytes alignment for allocation for use.
> > > > > 
> > > > >    include/linux/genalloc.h | 23 +++++++++++++++----
> > > > >    lib/genalloc.c           | 58
> > > > +++++++++++++++++++++++++++++++++++++++++++-----
> > > > >    2 files changed, 72 insertions(+), 9 deletions(-)
> > > > > 
> > > > > diff --git a/include/linux/genalloc.h b/include/linux/genalloc.h
> > > > > index 1ccaab4..55da07e 100644
> > > > > --- a/include/linux/genalloc.h
> > > > > +++ b/include/linux/genalloc.h
> > > > > @@ -34,6 +34,7 @@
> > > > > 
> > > > >    struct device;
> > > > >    struct device_node;
> > > > > +struct gen_pool;
> > > > > 
> > > > >    /**
> > > > >     * Allocation callback function type definition @@ -47,7 +48,7 @@
> > > > > typedef unsigned long (*genpool_algo_t)(unsigned long *map,
> > > > >                       unsigned long size,
> > > > >                       unsigned long start,
> > > > >                       unsigned int nr,
> > > > > -                     void *data);
> > > > > +                     void *data, struct gen_pool *pool);
> > > > > 
> > > > >    /*
> > > > >     *  General purpose special memory pool descriptor.
> > > > > @@ -73,6 +74,13 @@ struct gen_pool_chunk {
> > > > >       unsigned long bits[0];          /* bitmap for allocating memory
> > chunk
> > > > */
> > > > >    };
> > > > > 
> > > > > +/*
> > > > > + *  gen_pool data descriptor for gen_pool_first_fit_align.
> > > > > + */
> > > > > +struct genpool_data_align {
> > > > > +     int align;              /* alignment by bytes for starting address */
> > > > > +};
> > > > > +
> > > > 
> > > > (sorry for chiming in late, I've been traveling)
> > > > 
> > > > Is there an advantage here to wrapping this in a structure instead of
> > > > just passing a pointer to an align integer?
> > > 
> > > 
> > > Please look at the commit message for
> > > ca279cf1065fb689abea1dc7d8c11787729bb185 which adds "data":
> > > 
> > > "As I can't predict all the possible requirements/needs for all
> > > allocation uses cases, I add a "free" field 'void *data' to pass any
> > > needed information to the allocation function.  For example 'data'
> > > could be used to handle a structure where you store the alignment, the
> > > expected memory bank, the requester device, or any information that
> > > could influence the allocation algorithm."
> > > 
> > 
> > Right, I understand what the purpose is but I'm not sure what you're
> > getting from the structure vs passing a pointer, e.g.
> > 
> > int align;
> > 
> > align = 4;
> > 
> > gen_pool_alloc_data(&pool, size, &align);
> > 
> > it just seems to obfuscate what's going on by wrapping a single integer
> > in a structure that's narrowly defined in a generic function right now. I
> > guess it could change later which would necessitate having the structure
> > but again it's so generic I'm not sure what else you would pass that
> > would be applicable to all clients.
> 
> Scott and me have discussed about this issue in my RFC patch.
> Please review: http://patchwork.ozlabs.org/patch/493297/

I don't see anything relevant in that discussion.  I tend to favor always 
using a struct for this type of opaque data, for consistency and 
extendability, but in this case it really doesn't matter much either way.

-Scott