linux-kernel - Re: [PATCH] bsg : Add support for io vectors in bsg

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Message-ID: <20080110204325.GC1928@osc.edu>
Date:	Thu, 10 Jan 2008 15:43:25 -0500
From:	Pete Wyckoff <pw@....edu>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
Cc:	tomof@....org, deepakrc@...il.com, linux-scsi@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bsg : Add support for io vectors in bsg

fujita.tomonori@....ntt.co.jp wrote on Wed, 09 Jan 2008 09:11 +0900:
> On Tue, 8 Jan 2008 17:09:18 -0500
> Pete Wyckoff <pw@....edu> wrote:
> > I took another look at the compat approach, to see if it is feasible
> > to keep the compat handling somewhere else, without the use of #ifdef
> > CONFIG_COMPAT and size-comparison code inside bsg.c.  I don't see how.
> > The use of iovec is within a write operation on a char device.  It's
> > not amenable to a compat_sys_ or a .compat_ioctl approach.
> > 
> > I'm partial to #1 because the use of architecture-independent fields
> > matches the rest of struct sg_io_v4.  But if you don't want to have
> > another iovec type in the kernel, could we do #2 but just return
> > -EINVAL if the need for compat is detected?  I.e. change
> > dout_iovec_count to dout_iovec_length and do the math?
> 
> If you are ok with removing the write/read interface and just have
> ioctl, we could can handle comapt stuff like others do. But I think
> that you (OSD people) really want to keep the write/read
> interface. Sorry, I think that there is no workaround to support iovec
> in bsg.

I don't care about read/write in particular.  But we do need some
way to launch asynchronous SCSI commands, and currently read/write
are the only way to do that in bsg.  The reason is to keep multiple
spindles busy at the same time.

How about these new ioctls instead of read/write:

    SG_IO_SUBMIT - start a new blk_execute_rq_nowait()
    SG_IO_TEST   - complete and return a previous req
    SG_IO_WAIT   - wait for a req to finish, interruptibly

Then old write users will instead do ioctl SUBMIT.  Read users will
do TEST for non-blocking fd, or WAIT for blocking.  And SG_IO could
be implemented as SUBMIT + WAIT.

Then we can do compat_ioctl and convert up iovecs out-of-line before
calling the normal functions.

Let me know if you want a patch for this.

		-- Pete
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