Subject: [RFC PATCH dtc] C-based DT schema checker integrated into dtc

From: [email-masked] (Arnd Bergmann)
Date: Tue, 5 Nov 2013 21:17:32 +0100

On Tuesday 05 November 2013, Jason Gunthorpe wrote:
> On Tue, Nov 05, 2013 at 08:34:00PM +0100, Arnd Bergmann wrote:
> > On Tuesday 05 November 2013, Jason Gunthorpe wrote:
> > > The issue is sysfs, the TPM core attaches attributes to the driver's
> > > struct device, which means it has to convert from struct device * to
> > > its own private data. It is totally wrong, but that is the way
> > > it has been for ever. :(
> > 
> > I've just had a very brief look at that subsystem, and the first driver
> > I looked at (tpm_atmel) actually creates a child platform_device for
> > the sole purpose of registering that with the tpm subsystem, which seems
> > to avoid this problem. 
> 
> The TPM subsystem requires a struct device to bind onto (for sysfs at
> least), so drivers that don't have one are required to create one :|
> 
> atmel is unusual as most drivers have a pre-existing device.

Eeek, yes I see now. OTOH, it would still be able to do the same thing
by binding to the actual platform_device. Judging from the copyright
notice, this driver predates the move to creating 'struct device'
instances for (most) device nodes, and it uses the old-style
method of searching the DT from the module_init function, which is
not how we do things today.

Creating a platform_device (or a plain device, for that matter)
from a proper probe() function would have a similar effect from
the perspective of the TPM subsystem (or other subsystems using
the same drvdata trick), and make it possible to use my proposed
devm_probe method from any tpm driver. The main downside I can
see is that it requires duplicating some code in each TPM driver
that wants to convert to devm_probe, but on the upside it doesn't
require any changes to the TPM subsystem or to other drivers the
way that a proper fix would.

> > Maybe the same can be done for the other drivers as
> > well. Unfortunately, that will change the sysfs structure, which
> > might break user space relying on the current path to the
> > device. 
> 
> AFAIK the correct fix is to have the TPM subsystem core create a
> struct device for its own use (eg tpm0), under its own class, which is
> what other subsystems I looked at do.

Yes. Note that the use of new 'class'es is not recommended any more,
nowadays we are supposed to use 'bus_type' for this, which is
traditionally something slightly different, but technically almost
the same implementation-wise.

> This way the subsystem can have
> access to a release function that is properly tied to the actual
> object lifetime, and it can use container_of to retrieve its own
> private data from, eg sysfs. Combined with get_ops/put_ops style
> access I think you can solve the unload lifetime issues with sysfs..
> 
> However, even if all that is done, compatibility sysfs's under the
> driver's struct device will still be needed, which AFAICT will still
> require the drvdata be owned by the subsystem not the driver :(

That depends: the requirement is that no user space breaks after
a change. I assume that there is a fairly limited set of packages
accessing the tpm sysfs files. If all of them are written properly,
they should be able to deal with looking at the files in a different
place in sysfs.

Another option is to change the TPM core sysfs attributes not to
look at drvdata, but to use devres_find() to find the data they
need.

	Arnd