Re: [PATCH] scsi: sr.c use unaligned access helpers

[Matthew Wilcox <matthew@wil.cx>]

On Tue, Oct 07, 2008 at 12:25:31PM -0700, Andrew Morton wrote:
> On Tue, 07 Oct 2008 14:19:14 +0200
> Boaz Harrosh <bharrosh@panasas.com> wrote:
> > I feel the opposite. That is: put_unaligned_be32(block, SCpnt->cmnd + 2);
> > is much more readable for me. Coming from the spec, I'm looking for a __b32
> > at offset CDB+2 and not: "SCpnt->cmnd[4] = (unsigned char) (block >> 8) & 0xff;"
> > At offset CDB+4 the 2nd-or-3rd? order byte of "block".
> > 
> > And for BE systems it's a gain. So please DO
> 
> Yeah.  For neophytes it's a good change.  This:
> 
> 	SCpnt->cmnd[2] = (unsigned char) (block >> 24) & 0xff;
> 	SCpnt->cmnd[3] = (unsigned char) (block >> 16) & 0xff;
> 	SCpnt->cmnd[4] = (unsigned char) (block >> 8) & 0xff;
> 	SCpnt->cmnd[5] = (unsigned char) block & 0xff;
> 
> is "wtf is that doing?", whereas this:
> 
> 	put_unaligned_be32(block, SCpnt->cmnd + 2);
> 
> is "ah, I know what that's doing".

And scsi_put_u32(block, SCpnt->cmnd + 2); is even more obvious.  You
don't even need to know that SCSI is a big-endian protocol that way,
just that the u32 is being written in the appropriate way for SCSI
(see also ntohl et al).

Also, SCSI needs to be able to read/write 'u24' quantities, which is
potentially an ambiguous thing if you consider it in terms of
byteswapping.  I don't think we want a put_unaligned_be24() function,
but we do want a scsi_put_u24().

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."