linux-kernel - Re: [PATCH v5 7/7] tpm: create TPM 2.0 devices using own device class

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Date:	Tue, 4 Nov 2014 22:38:43 +0200
From:	Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
Cc:	Peter Huewe <peterhuewe@....de>, Ashley Lai <ashley@...leylai.com>,
	Marcel Selhorst <tpmdd@...horst.net>,
	tpmdd-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
	josh.triplett@...el.com, christophe.ricard@...il.com,
	jason.gunthorpe@...idianresearch.com
Subject: Re: [PATCH v5 7/7] tpm: create TPM 2.0 devices using own device class

On Tue, Nov 04, 2014 at 11:14:33AM -0700, Jason Gunthorpe wrote:
> On Tue, Nov 04, 2014 at 01:47:34PM +0200, Jarkko Sakkinen wrote:
> 
> > > > I used the class 'tpm' only for TPM 2.x because I didn't want to
> > > > break the binary compatibility for TPM 1.x anyway. In ideal situtation
> > > > both would be character devices inside the class 'tpm' and there would
> > > > be sysfs attribute such as 'family' to mark the protocol to be used.
> > > 
> > > You can create the class without moving away from miscdev...
> > > 
> > > Not having the device creates way to much difference that has to be
> > > supported, way too messy.
> > 
> > I have to admit that I'm not quite following here but I assume that
> > you restated this below in more verbose way and this is basically the
> > same argument :)
> 
> I mean, if we have a patch that does:
> 
> struct tpm_chip {
>    struct device cdev;  // the class device
>    struct device *pdev; // the 'platform' device chip is bound too
> 
>    struct device *dev = pdev; // Temporary Compatability
> [+ device_register/etc/etc]
> 
> Then both cdev and pdev should always be valid. We should not have cdev
> be valid for TPM2 and invalid for TPM1, that is just a big mess.
> 
> Just this change is perfectly safe, it creates the /sys/class/tpm/tpm0
> directory, but doesn't change anything already existing
> 
> Then, a patch: go through and replace all uses of chip->dev with &chip->cdev in
> 90% of cases
> 
> Another patch: use chip->pdev for the handfull of the rest, and drop
> dev entirely
> 
> Then a patch: Drop misc_register entirely, no compat stuff. Explain
> clearly the resulting sysfs changes, CC the various people who monitor
> the sysfs API, act on any feedback. I'm hoping it is an OK change.
> [ If it is not OK then we can talk about using it only for TPM2 or
>   whatever ]

Hold on. So you are proposing that for a TPM 1.0 device you would
have simultaneously as an intermediate step:

- /sys/class/tpm/tpm0
- /sys/class/misc/tpm0

Maybe it would be a better idea to just create patch that would simply

- wipe /sys/class/misc/tpm0
- introduce /sys/class/tpm/tpm0
- for TPM 1 put existing sysfs attributes /sys/class/tpm/tpm0

I would put this into a  single patch. I don't really like those 
intermediate steps. I think they cause more confusion but I think
as a whole the change is reasonable.

I'll do this "aggressive" patch for v6 and "soften" it based on 
feedback if it is not liked (and CC to appropriate people).

> > I think the current variable name "dev" is miss-leading. The
> > use of "pdev" would document better the appropriate use for that
> > field (i.e. for the most cases DON'T use it).
> 
> Yes, see above :)
> 
> > I think that would be a mess. The way things are done in this v5
> > patch set is actually quite coherent and it does not break backwards
> > compatibility because the "proper" device hierarchy is only used for
> > TPM 2.0 devices.
> 
> I mean, just do something like this:
> 
> if (chip->id == 0)
>    chip->cdev.devt = MKDEV(MISC_MAJOR, TPM_MINOR)
> else
>    chip->cdev.devt = [.. dynamic alloc_chrddev_region ..]
> 
> Very simple, keeps the major/minor for TPM0, moves the 'dev' file to
> the new location in sysfs.
> 
> I can't find another example of this in the kernel, so I don't know
> what the thinking is..

Maybe it would be reasonable to break the path structure and do this
change for the benefit of backward compatibility. Not doing the change
your proposed might really cause unwanted breakage.

Moving the current TPM sysfs attributes is not really an issue because
they are only for human consumption (not machine readable in any sany
way) :) And AFAIK TrouSerS does not use sysfs PPI interface.

> Jason

/Jarkko
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