linux-kernel - Re: [PATCH] tpm: unified PPI interface for TPM 1.x/2.0 devices

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Date:	Wed, 8 Apr 2015 10:26:07 +0300
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,
	christophe.ricard@...il.com, jason.gunthorpe@...idianresearch.com,
	stefanb@...ux.vnet.ibm.com
Subject: Re: [PATCH] tpm: unified PPI interface for TPM 1.x/2.0 devices

On Wed, Apr 01, 2015 at 12:19:25PM -0600, Jason Gunthorpe wrote:
> On Wed, Apr 01, 2015 at 03:28:52PM +0300, Jarkko Sakkinen wrote:
> > Added PPI interface to the character device. PPI interface is also kept
> > in the pdev for backwards compatibility.
> 
> Could you look at just completely moving the PPI interface to the char
> dev and then adding a symlink from the pdev? That would be really
> ideal.
> 
> symlinks have the advantage that they actually fully fix the lifetime
> issues.
> 
> This seems doable, if we replace the ppi_attrs group with a bunch of
> calls to sysfs_create_link it should work ?

If we follow the pattern in [1] by the book, how would you use
sysfs_create_link()? To be more specific, how would you get the driver
core to create the symlinks for you? 

If we decide not to follow [1] by the book, then this might be doable 
(thinking off my head, that's the reason why I use *might be* instead
of *is*). Wouldn't we get non-racy behavior if sysfs_create_link()'s
are executed after device_initialize() and before device_add()?

> > +static struct tpm_chip *ppi_dev_to_chip(struct device *dev)
> > +{
> > +	struct tpm_chip *chip = dev_get_drvdata(dev);
> > +
> > +	if (chip == NULL)
> > +		chip = to_tpm_chip(chip);
> > +
> > +	return chip;
> > +}
> 
> If symlinks don't work out, we should probably just set the drvdata on
> the tpm_chip itself to avoid this.

I'll experiment with this. Thanks for the comment.

> > +	if (!(chip->flags & TPM_CHIP_FLAG_PPI))
> > +		return -EINVAL;
> 
> Hum, I don't think the PPI files should be created if there is no PPI
> support..

Again, following [1] by the book. And again, I think we could just as
well do our sysfs stuff in-between device_initialize() and device_add()
and get the non-racy behavior.

> > +void __init tpm_ppi_init(struct class *tpm_class)
> > +{
> > +	tpm_class->dev_groups = tpm_groups;
> >  }
> 
> So this shouldn't be unconditional.
> 
> Also, ultimately PPI can't just claim the dev_groups, other parts of
> the driver will need to add groups too.
> 
> I think it makes more sense to do
> 
> struct attribute_group *tpm_ppi_get_sysfs(struct tpm_chip *chip)
> {
> }
> 
> And take care of building the list in the caller.
> 
> And tpm_ppi_get_sysfs should be called after the driver is readied but
> before adding the device.

I don't think this would matter. Things could be refactored when more
sysfs attributes are needed.

> Jason

/Jarkko


[1] http://kroah.com/log/blog/2013/06/26/how-to-create-a-sysfs-file-correctly/

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