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

[Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>]

On Tue, Nov 04, 2014 at 01:47:34PM +0200, Jarkko Sakkinen wrote:
> On Mon, 2014-11-03 at 14:38 -0700, Jason Gunthorpe wrote:
> > On Mon, Nov 03, 2014 at 07:41:01AM +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 :)
> 
> > > > And considering the volume of changes it might be better to leave
> > > > 'dev' as a pointer to the tpm class rather than try and tackle that in
> > > > this giant patch..
> > > 
> > > Maybe, or maybe I could make the rename a separate patch?
> > 
> > Pointer then rename?
> > 
> > > It's fairly mechanical, just a matter of replacing string
> > > "chip->dev" with "chip->pdev".
> > 
> > Not everyone should be replaced :|
> 
> 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).
> 
> > > > > +	chip->cdev.owner = chip->pdev->driver->owner;
> > > > 
> > > > Is that right? the cdev fops is in this module, not the driver's
> > > > module..
> > > 
> > > tpm.ko defines the interface but TPM device driver module owns the
> > > character device. I think this is right and similar logic is also
> > > for example rtc_device_register().
> > 
> > Hmm, yes, I see that in rtc_dev_prepare - but I don't have time to
> > figure out why that might be :)
> > 
> > On the surface, it doesn't seem to make sense: The cdev layer never
> > calls into a function that goes to the chip module - all functions go
> > to the fops in tpm.ko, and tmp.ko is (eventually) responsible for
> > ensuring that ops never points to an unloaded module.
> > 
> > So why should it care what the driver module is??
> 
> Fully agreed, I think you got a point here. I think it makes sense
> as a guideline to kind of "centralize" robustness to tpm.ko and
> leave as little responsibility as possible to device drivers.
> 
> > > I could understand in the context of a misc device but don't really
> > > when TPM 2.0 devices have their own device class. Using a 'tpm' class
> > > would in all cases break non-udev systems because major number is no
> > > longer 10 (misc).
> > 
> > I'm just saying it would be nice to force the major/minor to misc.tpm
> > for tmp0.
> > 
> > No idea if that is OK or not.
> 
> 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 think the improvement for v6 would be to add a sysctl attribute
> to disable legacy device hierarchy and sysfs attributes. If that
> attribute is unset, the old misc hierarchy would be used for any
> TPM version and TPM 1.0 devices would use existing sysfs attributes.

Mistake here. Should be a module parameter to be not racy, not 
sysctl attribute :)

> If the flag is set, then the new proper device hierarchy would be
> used for any TPM device and we would have also chance to redefine
> sysfs attributes.
> 
> This would add some legacy clutter to the stack but would be a
> piss-off-free and non-ABI-breaking solution.
> 
> > Jason
> 
> PS. I feel that drivers/char/tpm is a like a desolated town. How 
> many of the people currently marked as maintainers are actually
> participating to the subsystem development let alone driving it?
> 
> This subsystem is increasingly important to us given that major
> software platforms like Chrome OS make extensive use of it and
> I've heard that even bug fixes have taken some times over a year
> to get through. Does not look good at all.

/Jarkko