Baolin Wang <baolin.wang@xxxxxxxxxx> writes:
> +/**
> + * sg_is_contiguous - Check if the scatterlists are contiguous
> + * @sga: SG entry
> + * @sgb: SG entry
> + *
> + * Description:
> + * If the sga scatterlist is contiguous with the sgb scatterlist,
> + * that means they can be merged together.
> + *
> + **/
> +static inline bool sg_is_contiguous(struct scatterlist *sga,
> + struct scatterlist *sgb)
> +{
> + return *(unsigned long *)sg_virt(sga) + sga->length ==
> + *(unsigned long *)sg_virt(sgb);
As I already said, I don't like casts.
But let's take some height : you're needing this function to decide to merge
scatterlists. That means that you think the probability of having 2 scatterlist
mergeable is not meaningless, ie. 50% or more.
I suppose your scatterlists are allocated through kmalloc(). I'd like to know,
through your testing, what is the success rate of sg_is_contiguous(), ie. I'd
like to know how many times sg_is_contiguous() was called, and amongst these
calls how many times it returned true.
That will tell me how "worth" is this optimization.
> + * 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 mapped scatterlists has been added
> + * in the dynamic sg table. The 'orig_nents' member indicates the size of the
> + * dynamic sg table.
> + *
> + * Copy one mapped @src@ scatterlist into the dynamic sg table and increase
> + * 'nents' member.
> + *
> + **/
Okay, I still believe this one is wrong, because we don't understand each other.
So let's take an example :
sg_table = {
.sgl = {
{
.page_link = PAGE_48,
.offset = 0,
.length = 2048,
.dma_address = 0x30000,
.dma_length = 4096,
},
{
.page_link = PAGE_48 | 0x02,
.offset = 2048,
.length = 2048,
.dma_address = 0,
.dma_length = 0,
},
},
.nents = 1,
.orig_nents = 2,
};
In this example, by sheer luck the 2 scatterlist entries were physically
contiguous, and the mapping function coallesced the 2 entries into only one
(dma_address, dma_length) entry. That could also happen with an IOMMU by the
way. Therefore, sg_table.nents = 1.
If I understand your function correctly, it will erase sg_table.sgl[1], and that
looks incorrect to me. This is why I can't understand how your code can be
correct, and why I say you add a new "meaning" to sg_table->nents, which is not
consistent with the meaning I understand.
Cheers.
--
Robert
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