From: Pete Wyckoff <pw@xxxxxxx>
Subject: [PATCH] bsg: iovec support
Date: Thu, 1 Mar 2007 17:29:08 -0500
> Support vectored IO as in SGv3. The iovec structure uses explicit
> sizes to avoid the need for compat conversion.
>
> Signed-off-by: Pete Wyckoff <pw@xxxxxxx>
> ---
>
> My application definitely can take advantage of scatter/gather IO,
> which is supported in sgv3 but not in the bsg implementation of sgv4.
> I understand Tomo's concerns about code bloat and the need for
> 32/64 compat translations, but this will make things much easier on
> users of bsg who read or write out of multiple buffers in a single
> SCSI operation.
(snip)
> + * Vector of address/length pairs, used when dout_iovec_count (or din_)
> + * is non-zero. In that case, dout_xferp is a list of struct sg_io_v4_vec
> + * and dout_iovec_count is the number of entries in that list. dout_xfer_len
> + * is the total length of the list. Note the use of u64 instead of a
> + * native pointer to avoid compat issues, and padding to avoid structure
> + * alignment problems.
> + */
> +struct sg_io_v4_vec {
> + __u64 iov_base;
> + __u32 iov_len;
> + __u32 __pad1;
> +};
I don't think that it's a good idea to add a new scatter/gather
structure and export it to user space.
bsg can support scatter/gather IO with ioctl (SG_IO) easily (I mean,
without adding ugly compat code to bsg.c). I guess that SG_IO doesn't
work for you because it works synchronously. However, all system calls
might work asynchronously in the future.
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