kern/52338: fd(4) floppy disk driver & non-blocking I/O

[Bruce Evans]

On Mon, 19 May 2003, Yar Tikhiy wrote:

> On Sun, May 18, 2003 at 12:39:03AM +1000, Bruce Evans wrote:
> >
> > It should set bp->bio_resid (to bp->bio_bcount) (bio_resid and bio_bcount
> > are the same as b_resid and b_bcount here; strategy routines only have
> > access to a struct bio so they must use the former).
>
> What do you think about the following straightforward patch to fd.c?
> It catches all spots where BIO_ERROR is set, but bio_resid isn't.
> In fd.c, bio_resid is never set in advance, so the simplest approach
> should work.
>
> OTOH, I wonder if bio_resid could be set equal to bio_bcount at the
> beginning of fdstrategy() and changed only if needed.  Does this have
> any obscure implications?

I think that is the right place to set it.  The residual count really
is the full count initially.  Any obfuscations come later.  The driver
might prefer to only set bio_resid to its final value at the end of
normal i/o instead of as every block is finished.  Then the initial
setting would only used for abnormal i/o.  This is essentially what
the fd driver does (except it just forgets to set bio_resid for abnormal
i/o).

> And my other thought is:  What if physio() sets bio_resid equal to
> bio_bcount before calling DEV_STRATEGY()?  Currently, physio()
> leaves bio_resid unset.  An obvious drawback of this approach is
> that it would encourage poor coding in drivers, though.

Rev.1.37 of kern/subr_disk.c sets bio_resid to bio_bcount in the generic
strategy routine.  This is a better place than physio().  The fd driver
didn't benefit from this because it never used the disk mini-layer.
Now geom is used instead of the disk mini-layer.  geom now uses
essentially the same method as the fd driver for normal i/o -- it keeps
track of the amount of i/o completed and subtracts this from bio_bcount
to get bio_resid when i/o completes, and it doesn't set bio_resid
initially.  The fd driver doesn't benefit from this because it doesn't
use geom either.

> --- fd.c.dist	Fri Apr 11 15:39:24 2003
> +++ fd.c	Mon May 19 21:48:11 2003
> @@ -1668,8 +1668,9 @@ fdstrategy(struct bio *bp)
>  		      (u_long)major(bp->bio_dev), (u_long)minor(bp->bio_dev));
>  	fdc = fd->fdc;
>  	if (fd->type == FDT_NONE || fd->ft == 0) {
> -		bp->bio_error = ENXIO;
> +		bp->bio_error = fd->type == FDT_NONE ? ENXIO : EAGAIN;
>  		bp->bio_flags |= BIO_ERROR;
> +		bp->bio_resid = bp->bio_bcount;
>  		goto bad;
>  	}
>  	fdblk = 128 << (fd->ft->secsize);

I think this should check FD_NONBLOCK like the next clause does, and only
return EAGAIN when FD_NONBLOCK is set.  This is clearer even if it is
equivalent (can we get here with a type but no ft?).

Bruce