linux-kernel - Re: [PATCH 14/21] MSI: Use a list instead of the custom link structure

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Message-Id: <1175144609.16660.33.camel@concordia.ozlabs.ibm.com>
Date:	Thu, 29 Mar 2007 15:03:29 +1000
From:	Michael Ellerman <michael@...erman.id.au>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	linux-pci@...ey.karlin.mff.cuni.cz,
	Greg Kroah-Hartman <greg@...ah.com>,
	"David S. Miller" <davem@...emloft.net>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	linux-kernel@...r.kernel.org, Andrew Morton <akpm@...l.org>,
	daniel.e.wolstenholme@...el.com
Subject: Re: [PATCH 14/21] MSI: Use a list instead of the custom link
	structure

On Wed, 2007-03-28 at 00:29 -0600, Eric W. Biederman wrote:
> Michael Ellerman <michael@...erman.id.au> writes:
> 
> > The msi descriptors are linked together with what looks a lot like
> > a linked list, but isn't a struct list_head list. Make it one.
> >
> > The only complication is that previously we walked a list of irqs, and
> > got the descriptor for each with get_irq_msi(). Now we have a list of
> > descriptors and need to get the irq out of it, so it needs to be in the
> > actual struct msi_desc. We use 0 to indicate no irq is setup.
> >
> > At some point after a pci_dev is created we need to initialise its
> > msi_list. pci_device_add() looks like the right place to do that, although
> > I'm not convinced it's 100% safe. In drivers/char/agp/alpha-agp.c we create
> > a pci_dev and I don't see that it ever gets passed to pci_device_add(), but
> > we probably don't care.
> 
> Well that one appears to be a dummy place holder and probably should at
> least have a kzalloc to initialize all of the fields to know values.
> 
> Regardless the normal pci device allocation does use kzalloc so we will
> have well defined if not beautiful behavior if we try and use it.
> 
> Until we have a case where we need to use the msi_list outside of 
> where we enable and disable msi we should be perfectly fine initializing
> the list somewhere inside of pci_enable_msi, and pci_enable_msix.
> With dev->msi_enabled and dev->msix_enabled serving as flags to the
> rest of the world that it is safe to look at the list.
> 
> It certainly sounds safer to me then becoming to closely coupled with
> code that doesn't really care about how msi works.  Heck even though
> we repeat the call twice I bet it will even be less code.

I thought about doing it in the MSI enable methods, but I think it
really belongs in the (nonexistant) routine that allocs and sets up a
pci_dev.

I think it's pretty dicy to be passing around a pci_dev with an
uninitialised msi_list. Even if currently no code outside the MSI enable
methods looks at it, I think we're asking for bugs in the future.

So I'll do a patch which adds alloc_pci_dev(), update the callers, and
then put the msi_list initialisation in there.

> > --- msi-new.orig/include/linux/msi.h
> > +++ msi-new/include/linux/msi.h
> > @@ -1,6 +1,8 @@
> >  #ifndef LINUX_MSI_H
> >  #define LINUX_MSI_H
> >  
> > +#include <linux/list.h>
> > +
> >  struct msi_msg {
> >  	u32	address_lo;	/* low 32 bits of msi message address */
> >  	u32	address_hi;	/* high 32 bits of msi message address */
> > @@ -24,10 +26,8 @@ struct msi_desc {
> >  		unsigned default_irq;	/* default pre-assigned irq	  */
> >  	}msi_attrib;
> >  
> > -	struct {
> > -		__u16	head;
> > -		__u16	tail;
> > -	}link;
> > +	int irq;
> This should be "unsigned int irq"

Oops, I'll fix that.

cheers

-- 
Michael Ellerman
OzLabs, IBM Australia Development Lab

wwweb: http://michael.ellerman.id.au
phone: +61 2 6212 1183 (tie line 70 21183)

We do not inherit the earth from our ancestors,
we borrow it from our children. - S.M.A.R.T Person

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