Subject: Re: [PATCH] Add TPM hardware enablement driver
From: Kylene Jo Hall <kjhall@us.ibm.com>
To: Greg KH <greg@kroah.com>
Cc: Jeff Garzik <jgarzik@pobox.com>, linux-kernel@vger.kernel.org
In-Reply-To: <1112717690.7713.23.camel@jo.austin.ibm.com>
References: <1110415321526@kroah.com> <422FC42B.7@pobox.com>
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	 <1112717690.7713.23.camel@jo.austin.ibm.com>
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Date: Fri, 08 Apr 2005 15:07:34 -0500
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On Tue, 2005-04-05 at 11:14 -0500, Kylene Jo Hall wrote:
> On Thu, 2005-03-24 at 13:33 -0800, Greg KH wrote:
> > On Thu, Mar 24, 2005 at 04:04:25PM -0500, Jeff Garzik wrote:
> > > Greg KH wrote:
> > > >On Tue, Mar 22, 2005 at 09:02:24PM -0500, Jeff Garzik wrote:
> > > >
> > > >>Kylene Hall wrote:
> > > >>
> > > >>>>what is the purpose of this pci_dev_get/put?  attempting to prevent 
> > > >>>>hotplug or
> > > >>>>something?
> > > >>>
> > > >>>
> > > >>>Seems that since there is a refernce to the device in the chip structure 
> > > >>>and I am making the file private data pointer point to that chip 
> > > >>>structure this is another reference that must be accounted for. If you 
> > > >>>remove it with it open and attempt read or write bad things will happen. 
> > > >>>This isn't really hotpluggable either as the TPM is on the motherboard.
> > > >>
> > > >>My point was that there will always be a reference -anyway-, AFAICS. 
> > > >>There is a pci_dev reference assigned to the pci_driver when the PCI 
> > > >>driver is loaded, and all uses by the TPM generic code of this pointer 
> > > >>are -inside- the pci_driver's pci_dev object lifetime.
> > > >
> > > >
> > > >Think of the following situation:
> > > >	- driver is bound to device.
> > > >	- userspace opens char dev node.
> > > >	- device is removed from the system (using fakephp I can do this
> > > >	  to _any_ pci device, even if it is on the motherboard.)
> > > >	- userspace writes to char dev node
> > > >	- driver attempts to access pci device structure that is no
> > > >	  longer present in memory.
> > > >
> > > >Because of this open needs to get a reference to the pci device to
> > > >prevent oopses, or the driver needs to be aware of "device is now gone"
> > > >in some other manner.
> > > 
> > > Thanks for explaining; agreed.
> > > 
> > > However, there appear to still be massive bugs in this area:
> > > 
> > > Consider the behavior of the chrdev if a PCI device has been
> > > unplugged.  It's still actively messing with the non-existent
> > > hardware, and never checks for dead h/w AFAICS.
> > 
> > I agree, the driver should be fixed to handle this properly.
> > 
> 

Basically, what I need to figure out is how to solve both issues
simultaneously.  I need to not register a pci_driver as I would be
taking over an ID that is not unique to my device as well as get the
hotplugging correct (which i don't know how to do with out a pci_remove
function).

Thanks,

> I have now played with the fakephp driver and have a better
> understanding of these interactions, but I still have questions.  With
> the current structure there is a problem because everything is
> "cleaned-up" with the tpm_remove function even if userspace has the
> device open when the tpm's slot is removed and then there are problems
> on subsequent reads/writes. The get/put didn't really stop this from
> happening.  Is it right to fix this by cleaning mostly up and placing a
> flag in the read/write path to check for this condition?
> 
> This problem actually becomes more complicated.  Since the TPM lives on
> the LPC bus and does not have it's own id we were in the process of
> converting the driver to not use a pci_driver structure at all like the
> example in drivers/char/watchdog/i8xx_tco.c.  This is desirable so that
> the driver does not claim the id and other drivers can still find their
> devices that also live on the LPC bus and thus share the same ID.
> Without a pci_driver structure there is no probe or remove functions and
> thus the driver is not alerted of the loss of hardware.  Any
> recommendations of how to handle this situation?
> 
> Thanks,
> Kylie
> 


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