Subject: Re: [PATCH] bsg : Add support for io vectors in bsg
From: James Bottomley <James.Bottomley@HansenPartnership.com>
To: Pete Wyckoff <pw@osc.edu>
Cc: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>, tomof@acm.org,
       deepakrc@gmail.com, linux-scsi@vger.kernel.org,
       linux-kernel@vger.kernel.org
In-Reply-To: <20080110204325.GC1928@osc.edu>
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	 <20080109091118N.fujita.tomonori@lab.ntt.co.jp>
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Date: Thu, 10 Jan 2008 14:55:31 -0600
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On Thu, 2008-01-10 at 15:43 -0500, Pete Wyckoff wrote:
> fujita.tomonori@lab.ntt.co.jp wrote on Wed, 09 Jan 2008 09:11 +0900:
> > On Tue, 8 Jan 2008 17:09:18 -0500
> > Pete Wyckoff <pw@osc.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.

Won't multi-threading the ioctl calls achieve the same effect?  Or do
you trip over BKL there?

> 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.

Really, the thought of re-inventing yet another async I/O interface
isn't very appealing.

James


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