Re: [PATCH 00/10] sg buffer copy helper functions

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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:

/*
 * 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?

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.

Alan Stern

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