Re: [PATCH 1/4] scatterlist: Introduce some helper functions

[Baolin Wang]

On 10 March 2016 at 17:42, Robert Jarzmik <robert.jarzmik@xxxxxxx> wrote:
>>
>>
>> Ah, sorry that's a mistake. It should check as below:
>> static inline bool sg_is_contiguous(struct scatterlist *sga, struct
>> scatterlist *sgb)
>> {
>>     return (unsigned int)sg_virt(sga) + sga->length == (unsigned
>> int)sg_virt(sgb);
>> }
> NAK.
> First, I don't like the cast.
> Second ask yourself: what if you compile on a 64 bit architecture ?

Sorry, it's a mistake. I've resend one email to correct that with
'unsigned long'.

> Third, I don't like void* arithmetics, some compilers might refuse it.

OK. I will modify that.

>
> And as a last comment, is it "virtual" contiguity you're looking after, physical
> contiguity, or dma_addr_t contiguity ?

Yes, I need check the virtual contiguity.

>
>>>> @@ -370,6 +370,65 @@ int sg_alloc_table(struct sg_table *table, unsigned int
>>>> nents, gfp_t gfp_mask)
>>> ...
>>>>  /**
>>>> + * sg_add_sg_to_table - Add one scatterlist into sg table
>>>> + * @sgt:     The sg table header to use
>>>> + * @src:     The sg need to be added into sg table
>>>> + *
>>>> + * Description:
>>>> + *   The 'nents' member indicates how many scatterlists added in the sg table.
>>>> + *   Copy the @src@ scatterlist into sg table and increase 'nents' member.
>>>> + *
>>>> + **/
>>>> +int sg_add_sg_to_table(struct sg_table *sgt, struct scatterlist *src)
>>>> +{
>>>> +     unsigned int i = 0, orig_nents = sgt->orig_nents;
>>>> +     struct scatterlist *sgl = sgt->sgl;
>>>> +     struct scatterlist *sg;
>>>> +
>>>> +     /* Check if there are enough space for the new sg to be added */
>>>> +     if (sgt->nents >= sgt->orig_nents)
>>>> +             return -EINVAL;
>>> I must admit I don't understand that one either : how do comparing the number of
>>> "mapped" entries against the number of "allocated" entries determines if there
>>> is enough room ?
>>
>> That's for a dynamic sg table. If there is one sg table allocated
>> 'orig_nents' scatterlists, and we need copy another mapped scatterlist
>> into the sg table if there are some requirements. So we use 'nents' to
>> record how many scatterlists have been copied into the sg table.
> As I'm still having difficulities to understand, please explain to me :
>  - what is a dynamic sg_table

OK. That's for some special usage. For example, the dm-crypt will send
one request mapped one scatterlist to engine level to handle the
request at one time. But we want to collect some requests together to
handle together at one time to improve engine efficiency. So in crypto
engine layer we need to copy the mapped scatterlists into one sg
table, which has been allocated some sg memory.

>  - how it works

The 'orig_nents' means how many scatterlists the sg table can hold.
The 'nents' means how many scatterlists have been copied into sg
table. So the 'nents' will be not equal as 'orig_nents' unless the sg
table is full.

>  - if it's "struct sg_table", how the field of this structure are different when
>    it's a dynamic sg_table from a "static sg_table" ?

>
>>>> +/**
>>>> + * sg_alloc_empty_table - Allocate one empty sg table
>>>> + * @sgt:     The sg table header to use
>>>> + * @nents:   Number of entries in sg list
>>>> + * @gfp_mask:        GFP allocation mask
>>>> + *
>>>> + *  Description:
>>>> + *    Allocate and initialize an sg table. The 'nents' member of sg_table
>>>> + *    indicates how many scatterlists added in the sg table. It should set
>>>> + *    0 which means there are no scatterlists added in this sg table now.
>>>> + *
>>>> + **/
>>>> +int sg_alloc_empty_table(struct sg_table *sgt, unsigned int nents,
>>>> +                      gfp_t gfp_mask)
>>> As for this one, there has to be a purpose for it I fail to see. From far away
>>> it looks exactly like sg_alloc_table(), excepting it "works around" the nents >
>>> 0 protection of __sg_alloc_table().
>>> What is exactly the need for this one, and if it's usefull why not simply
>>> changing the __sg_alloc_table() "nents > 0" test and see what the outcome of the
>>> review will be ?
>>
>> Like I said above. If we want to copy some mapped scatterlists into
>> one sg table, we should set the 'nents' to 0 to indicates how many
>> scatterlists coppied in the sg table.
> So how do you unmap this scatterlist once you've lost the number of mapped
> entries ?

I will not lost the number of mapped in sg table,  'nents' member
means how many mapped scatterlists in the dynamic table. Every time
one mapped sg copied into the dynamic table, the 'nents' will increase
1.

>
> I think we'll come back to this once I understand how a "dynamic" sg_table is
> different from a "static" one.
>
> Cheers.
>
> --
> Robert



-- 
Baolin.wang
Best Regards