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

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Date:	Mon, 24 Aug 2015 21:01:26 -0700
From:	Laura Abbott <labbott@...hat.com>
To:	Zhao Qiang <qiang.zhao@...escale.com>,
	Scott Wood <scottwood@...escale.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	"lauraa@...eaurora.org" <lauraa@...eaurora.org>,
	Xiaobo Xie <X.Xie@...escale.com>,
	"benh@...nel.crashing.org" <benh@...nel.crashing.org>,
	Li Leo <LeoLi@...escale.com>,
	"paulus@...ba.org" <paulus@...ba.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@...hat.com]
>> Sent: Tuesday, August 25, 2015 7:11 AM
>> To: Zhao Qiang-B45475; Wood Scott-B07421
>> Cc: linux-kernel@...r.kernel.org; linuxppc-dev@...ts.ozlabs.org;
>> lauraa@...eaurora.org; Xie Xiaobo-R63061; benh@...nel.crashing.org; Li
>> Yang-Leo-R58472; paulus@...ba.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@...escale.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.

Thanks,
Laura
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