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

[FUJITA Tomonori <tomof@acm.org>]

On Thu, 13 Mar 2008 14:32:59 -0400 (EDT)
Alan Stern <stern@rowland.harvard.edu> 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.