On Thu, 13 Mar 2008 14:32:59 -0400 (EDT)
Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
> On Thu, 13 Mar 2008, FUJITA Tomonori wrote:
>
> > > > + for_each_sg(*sgl, sg, nents, i) {
> > >
> > > Will there be a problem in subsequent calls if *sgl has been
> > > incremented but nents hasn't been changed? Maybe nents needs to be a
> > > pointer also.
> >
> > usb_stor_access_xfer_buf doesn't check scsi_sg_count (the number of sg
> > entries). It assumes that callers take care about the issue.
> >
> > If you want nents to be a pointer, I'm fine with it.
>
> If nents doesn't change then for_each_sg() won't work right. There
> could be an alternative macro:
Oops, I thought that for_each_sg is defined like:
#define for_each_sg(sglist, sg, nr, __i) \
for (__i = 0, sg = (sglist); __i < (nr) && sg; __i++, sg = sg_next(sg))
> /*
> * Loop over each sg element, stopping at the end of the chain
> */
> #define for_each_sg_all(sglist, sg, __i) \
> for (__i = 0, sg = (sglist); sg; __i++, sg = sg_next(sg))
>
> If you added this macro to include/linux/scatterlist.h and used it
> instead of for_each_sg() then you can get rid of nents entirely.
> However I'm not sure whether this would be safe. Do people sometimes
> use a subset of the entries in a scatterlist?
IIRC, some drivers do that (though they might use sg_next).
I don't think that we add a new macro just for this function. We could
change for_each_sg in the above way or we could just do in
usb_stor_access_xfer_buf
for (i = 0, sg = *sgl; i < nents && sg; i++, sg = sg_next(sg))
> If it isn't safe then nents would have to be passed as a pointer too.
> At this stage I think it would be better to encapsulate sgl, offset,
> and nents in a single structure than to pass multiple pointers.
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