async pass(4) patches available

[Konstantin Belousov]

On Mon, Mar 30, 2015 at 04:23:58PM -0600, Kenneth D. Merry wrote:
> Kernel memory for data transferred via the queued interface is 
> allocated from the zone allocator in MAXPHYS sized chunks, and user
> data is copied in and out.  This is likely faster than the
> vmapbuf()/vunmapbuf() method used by the CAMIOCOMMAND ioctl in
> configurations with many processors (there are more TLB shootdowns
> caused by the mapping/unmapping operation) but may not be as fast
> as running with unmapped I/O.
cam_periph_mapmem() uses vmapbuf() with an indicator to always map the
user pages mostly because I do not know CAM code and wanted to make
the least intrusive changes there.  It is not inherently impossible
to pass unmapped pages down from cam_periph_mapmem(), but might
require some more plumbing for driver to indicate that it is acceptable.

>         
> The new memory handling model for user requests also allows
> applications to send CCBs with request sizes that are larger than
> MAXPHYS.  The pass(4) driver now limits queued requests to the I/O
> size listed by the SIM driver in the maxio field in the Path
> Inquiry (XPT_PATH_INQ) CCB.
>         
> There are some things things would be good to add:
>         
> 1. Come up with a way to do unmapped I/O on multiple buffers.
>    Currently the unmapped I/O interface operates on a struct bio,
>    which includes only one address and length.  It would be nice
>    to be able to send an unmapped scatter/gather list down to
>    busdma.  This would allow eliminating the copy we currently do
>    for data.
Only because nothing more was needed.  The struct bio does not use
address/length pair when unmapped, it passes the list of physical
pages, see bio_ma array pointer.  It is indeed taylored to be a pointer
to struct buf' b_pages, but it does not have to be.

The busdma unmapped non-specific interface is bus_dmamap_load_ma(),
which again takes array of pages to load.  If you want some additional
helper, suitable for your goals, please provide the desired interface
definition.