Hi Christoph,
On 05.05.2020 13:09, Christoph Hellwig wrote:
> On Tue, May 05, 2020 at 12:51:58PM +0200, Marek Szyprowski wrote:
>> On 05.05.2020 12:15, Christoph Hellwig wrote:
>>>> - for_each_sg_page(st->sgl, &sg_iter, st->nents, 0)
>>>> + for_each_sg_page(st->sgl, &sg_iter, st->orig_nents, 0)
>>> Would it make sense to also add a for_each_sgtable_page helper that
>>> hides the use of orig_nents? To be used like:
>>>
>>> for_each_sgtable_page(st, &sg_iter, 0) {
>> We would need two helpers:
>>
>> for_each_sgtable_cpu_page() and for_each_sgtable_dma_page().
>>
>> I considered them, but then I found that there are already
>> for_each_sg_page(), for_each_sg_dma_page() and various special iterators
>> like sg_page_iter, sg_dma_page_iter and sg_mapping_iter. Too bad that
>> they are almost not used, at least in the DRM subsystem. I wonder if it
>> make sense to apply them or simply provide the two above mentioned
>> wrappers?
> None of the helpers helps with passing the right parameters from the
> sg_table. So in doube we'd need wrappers for all of the above, or
> none..
I've played a bit with the code and the existing scatterlist iterators -
for_each_sg_page() and for_each_sg_dma_page(). I've found them quite handy!
The biggest advantage of them is that they always iterate over
scatterlist in PAGE_SIZE units, what should make the code much easier to
understand. Here is example of their application to the function that
started this thread:
int drm_prime_sg_to_page_addr_arrays(struct sg_table *sgt, struct page
**pages,
dma_addr_t *addrs, int max_entries)
{
struct sg_dma_page_iter dma_iter;
struct sg_page_iter page_iter;
if (addrs)
for_each_sgtable_dma_page(sgt, &dma_iter, 0)
*addrs++ = sg_page_iter_dma_address(&dma_iter);
if (pages)
for_each_sgtable_page(sgt, &page_iter, 0)
*pages++ = sg_page_iter_page(&page_iter);
return 0;
}
After applying those iterators where possible (they can be used only for
reading the scatterlist), we would just need to add 2 trivial wrappers
to use them with sg_table objects:
#define for_each_sgtable_page(sgt, piter, pgoffset) \
for_each_sg_page(sgt->sgl, piter, sgt->orig_nents, pgoffset)
#define for_each_sgtable_dma_page(sgt, dma_iter, pgoffset) \
for_each_sg_dma_page(sgt->sgl, dma_iter, sgt->nents, pgoffset)
Then we would just need one more helper to construct scatterlist, as the
above two are read-only don't allow to modify scatterlist:
#define for_each_sgtable_sg(sgt, sg, i) \
for_each_sg(sgt->sgl, sg, sgt->orig_nents, i)
With the above 3 helpers we can probably get rid of all instances of
sg_table->{nents,orig_nents} from the DRM code. I will prepare patches soon.
Best regards
--
Marek Szyprowski, PhD
Samsung R&D Institute Poland
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