Re: [PATCH] iio: buffer-dmaengine: fix sg entry iteration when building dma_vecs

On Wed, Aug 19, 2026 at 01:41:04AM +0100, Jonathan Cameron wrote:
> On Tue, 18 Aug 2026 17:45:29 +0100
> Nuno Sá <[email protected]> wrote:

...

> >  		sgl = block->sg_table->sgl;
> > -		nents = sg_nents_for_len(sgl, block->bytes_used);
> > -		if (nents < 0)
> > -			return nents;
> > +		nents = sg_nents_for_dma(sgl, block->sg_table->nents, max_size);
> 
> So this fun function will generally give us the number of sgl entries, but not
> quite always. It will give us how many chunks of up to max_size fit into
> a particularly large entry.

It gives the number of SG entries needed for the case if each of them will
satisfy the limit. Whatever following code does, it may allocate a new SG
list based on the number returned by this function and resplit.

> > -		for (i = 0; i < nents; i++) {
> > +		for (i = 0; i < nents && len_total; i++) {
> So this needs to be more clever as we aren't just iterating entrees and filling
> them in, some of them could at least in theory be too big to fit
> in a single vec - hence you need to do a loop in here that sets
> multiple entries if that occurs.
> 
> If that can't happen for some other reason then I think you can 
> just use block->sgtable->nents instead of the more complex call above.

Perhaps sg_split() is what people are looking for in this case?

-- 
With Best Regards,
Andy Shevchenko