linux-kernel - Re: [PATCH] Add iSCSI IBFT Support (v0.3)

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Date:	Wed, 28 Nov 2007 15:21:40 -0400
From:	darnok@....org
To:	Greg KH <greg@...ah.com>
Cc:	darnok@....org, linux-kernel@...r.kernel.org, pjones@...hat.com,
	konradr@...hat.com, konradr@...ux.vnet.ibm.com,
	randy.dunlap@...cle.com, hpa@...or.com, lenb@...nel.org,
	mike.anderson@...ibm.com, dwm@...tin.ibm.com
Subject: Re: [PATCH] Add iSCSI IBFT Support (v0.3)

On Tue, Nov 27, 2007 at 11:09:19AM -0800, Greg KH wrote:
> On Tue, Nov 27, 2007 at 02:09:50PM -0400, darnok@....org wrote:
> > On Mon, Nov 26, 2007 at 09:29:55PM -0800, Greg KH wrote:
> > > On Mon, Nov 26, 2007 at 11:23:31PM -0500, Konrad Rzeszutek wrote:
> > > > On Monday 26 November 2007 22:31:38 Greg KH wrote:
> > > > > > +#if defined(CONFIG_ISCSI_IBFT) || defined(CONFIG_ISCSI_IBFT_MODULE)
> > > > ..snip..
> > > > > > +static ssize_t find_ibft(void)
> > > > > > +{
> > > > ..snip..
> > > > > > +}
> > > > >
> > > > > What is a function (not even an inline one) doing in a .h file?
> > > > 
> > > > I was not sure where to put it. This function (find_ibft) is used by the 
> > > > setup_[32|64].c and the iscsi_ibft.c code. Randy suggested I put in .c file, 
> > > > but I am not sure exactly where? Should I make a new file in called 
> > > > libs/iscsi_ibft_helper.c ?
> > > 
> > > Put it in your .c file and make it a global function to be called by
> > > someone else if they need it.
> > 
> > If the kernel is built with CONFIG_ISCSI_IBFT=m, the
> > setup_[32|64],c code would depend on the 'find_ibft' symbol which is
> > in a module (in the iscsi_ibft.c), which is not available during
> > the bootup phase and not linked to vmlinuz.
> 
> Ah, then don't allow that :)
> 
> > This isn't an issue if the module is built with CONFIG_ISCSI_IBFT=y of
> > course.
> > 
> > Or did by 'your .c file' mean a new file in arch/x86/kernel directory?
> 
> I didn't realize an external file, outside of your changes, needed this
> function.  If it does, then perhaps you need to just place it elsewhere.

The fundamental problem is that 'find_ibft' ought to be available
from anywhere (or at least from the iscsi_ibft.c) so that the iscsi_ibft
module can be loaded on any platform. But on x86, it needs to be called
 from setup_[32|64].c because the IBFT can be located anywhere
between 512KB and 1MB - and the E820 does not neccesarily have to
exclude that region. Hence the patch I proposed implements a
'reserve_ibft_region' code which would reserve the region of memory
found by 'find_ibft' so that it can be preserved when iscsi_ibft
module is actually loaded.

It ends up that there are three consumers of 'find_ibft':
 a) the module itself (iscsi_ibft.c)
 b) setup_32.c
 c) setup_64.c

The first choice, which looked the most flexible, was to have it
in iscsi_ibft.h file.
The second one, which would inhibit the user from making iscsi_ibft
a module, would be to move it to iscsi_ibft.c and make it
EXPORT_SYMBOL(), but that seems wasteful from a memory foot-print
look.
A third option was to put in /lib, but that doesn't seem right - this
'find_ibft' code is specific to this module.

Of all the options, the cleanest looks to be the first choice :-(
(I am not trying to be obstinate here - I just can't think of any
other reasonable place).
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