Re: [Intel-gfx] [PATCH 3/4] lib/scatterlist: Introduce and export __sg_alloc_table_from_pages

Tvrtko Ursulin
2017-08-03T09:21:25Z

On 03/08/2017 00:01, Andrew Morton wrote:
> On Wed, 2 Aug 2017 14:06:39 +0100 Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com> wrote:
>
>>
>> Hi Andrew,
>>
>> We have a couple of small lib/scatterlist.c tidies here, plus exporting
>> the new API which allows drivers to control the maximum coalesced entry
>> as created by __sg_alloc_table_from_pages.
>>
>> I am looking for an ack to merge these three patches via the drm-intel tree.
>>
>>
>> ...
>>
>> On 27/07/2017 10:05, Tvrtko Ursulin wrote:
>>> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>>
>>> Drivers like i915 benefit from being able to control the maxium
>>> size of the sg coallesced segment while building the scatter-
>
> "coalesced"

Oops, I've had this in other patches in the series. Fixed now.

>>> gather list.
>>>
>>> Introduce and export the __sg_alloc_table_from_pages function
>>> which will allow it that control.
>>>
>>
>> ...
>>
>>> --- a/lib/scatterlist.c
>>> +++ b/lib/scatterlist.c
>>> @@ -370,14 +370,15 @@ int sg_alloc_table(struct sg_table *table, unsigned int nents, gfp_t gfp_mask)
>>> EXPORT_SYMBOL(sg_alloc_table);
>>>
>>> /**
>>> - * sg_alloc_table_from_pages - Allocate and initialize an sg table from
>>> - * an array of pages
>>> - * @sgt: The sg table header to use
>>> - * @pages: Pointer to an array of page pointers
>>> - * @n_pages: Number of pages in the pages array
>>> - * @offset: Offset from start of the first page to the start of a buffer
>>> - * @size: Number of valid bytes in the buffer (after offset)
>>> - * @gfp_mask: GFP allocation mask
>>> + * __sg_alloc_table_from_pages - Allocate and initialize an sg table from
>>> + * an array of pages
>>> + * @sgt: The sg table header to use
>>> + * @pages: Pointer to an array of page pointers
>>> + * @n_pages: Number of pages in the pages array
>>> + * @offset: Offset from start of the first page to the start of a buffer
>>> + * @size: Number of valid bytes in the buffer (after offset)
>>> + * @max_segment: Maximum size of a scatterlist node in bytes (page aligned)
>>> + * @gfp_mask: GFP allocation mask
>>> *
>>> * Description:
>>> * Allocate and initialize an sg table from a list of pages. Contiguous
>
> The Description doesn't describe how this differs from
> sg_alloc_table_from_pages(), although it doesn't seem terribly
> important.

Well spotted, I've fixed this as well.

>>> +
>>> +/**
>>> + * sg_alloc_table_from_pages - Allocate and initialize an sg table from
>>> + * an array of pages
>>> + * @sgt: The sg table header to use
>>> + * @pages: Pointer to an array of page pointers
>>> + * @n_pages: Number of pages in the pages array
>>> + * @offset: Offset from start of the first page to the start of a buffer
>>> + * @size: Number of valid bytes in the buffer (after offset)
>>> + * @gfp_mask: GFP allocation mask
>>> + *
>>> + * Description:
>>> + * Allocate and initialize an sg table from a list of pages. Contiguous
>>> + * ranges of the pages are squashed into a single scatterlist node. A user
>>> + * may provide an offset at a start and a size of valid data in a buffer
>>> + * specified by the page array. The returned sg table is released by
>>> + * sg_free_table.
>>> + *
>>> + * Returns:
>>> + * 0 on success, negative error on failure
>>> + */
>>> +int sg_alloc_table_from_pages(struct sg_table *sgt, struct page **pages,
>>> + unsigned int n_pages, unsigned int offset,
>>> + unsigned long size, gfp_t gfp_mask)
>>> +{
>>> + return __sg_alloc_table_from_pages(sgt, pages, n_pages, offset, size,
>>> + SCATTERLIST_MAX_SEGMENT, gfp_mask);
>>> +}
>
> Making this an inline would save a bunch or stack space in the callers?
>
> One could just add the new arg then run around and update the 10ish
> callers but I guess the new interface is OK.

On the first suggestion - there are other trivial wrappers in
lib/scatterlist.c which could benefit from the same treatment (move to
inline) so I opted not to do this in this patch but will send a follow up.

> Otherwise it's OK by me, please go ahead as proposed.

Thanks a lot!

Regards,

Tvrtko