Re: [PATCH v20 12/12] null_blk: add support for copy offload

[Nitesh Shetty]

On 22/05/24 10:52AM, Bart Van Assche wrote:
> On 5/21/24 07:46, Nitesh Shetty wrote:
> > On 20/05/24 04:42PM, Bart Van Assche wrote:
> > > On 5/20/24 03:20, Nitesh Shetty wrote:
> > > > +    __rq_for_each_bio(bio, req) {
> > > > +        if (seg == blk_rq_nr_phys_segments(req)) {
> > > > +            sector_in = bio->bi_iter.bi_sector;
> > > > +            if (rem != bio->bi_iter.bi_size)
> > > > +                return status;
> > > > +        } else {
> > > > +            sector_out = bio->bi_iter.bi_sector;
> > > > +            rem = bio->bi_iter.bi_size;
> > > > +        }
> > > > +        seg++;
> > > > +    }
> > >
> > > _rq_for_each_bio() iterates over the bios in a request. Does a copy
> > > offload request always have two bios - one copy destination bio and
> > > one copy source bio? If so, is 'seg' a bio counter? Why is that bio
> > > counter compared with the number of physical segments in the request?
> > Yes, your observation is right. We are treating first bio as dst and
> > second as src. If not for that comparision, we might need to store the
> > index in a temporary variable and parse based on index value.
>
> I'm still wondering why 'seg' is compared with blk_rq_nr_phys_segments(req).
In this case blk_rq_nr_phys_segments is used as counter value(==2), which tells
its a src IO. But using a macro instead of this comparison will avoid this
confusion. We will change this in next version to make it explicit.

Thank you,
Nitesh Shetty