Re: [PATCH v3 2/8] genirq: irqdomain: Remove irqdomain dependency on struct device_node

["Rafael J. Wysocki" <rjw@rjwysocki.net>]

On Tuesday, September 15, 2015 10:18:32 AM Marc Zyngier wrote:
> On 15/09/15 00:15, Rafael J. Wysocki wrote:
> > On Monday, September 14, 2015 05:44:01 PM Marc Zyngier wrote:
> >> struct device_node is very much DT specific, and the original authors
> >> of the irqdomain subsystem recognized that tie, and went as far as
> >> mentionning that this could be replaced by some "void *token",
> >> should another firmware infrastructure be using it.
> >>
> >> As we move ACPI on arm64 towards this model too, it makes a lot of sense
> >> to perform that particular move.
> >>
> >> We replace "struct device_node *of_node" with "void *domain_token", which
> >> is a benign enough transformation. A non DT user of irqdomain can now
> >> identify its domains using this pointer.
> >>
> >> Also, in order to prevent the introduction of sideband type information,
> >> only DT is allowed to store a valid kernel pointer in domain_token
> >> (a pointer that passes the virt_addr_valid() test will be considered
> >> as a valid device_node).
> >>
> >> non-DT users that wish to store valid pointers in domain_token are
> >> required to use another structure such as an IDR.
> >>
> >> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
> >> ---
> >>  include/linux/irqdomain.h | 68 +++++++++++++++++++-----------------
> >>  kernel/irq/irqdomain.c    | 89 ++++++++++++++++++++++++++++++++++-------------
> >>  2 files changed, 101 insertions(+), 56 deletions(-)
> >>
> >> diff --git a/include/linux/irqdomain.h b/include/linux/irqdomain.h
> >> index f644fdb..ac7041b 100644
> >> --- a/include/linux/irqdomain.h
> >> +++ b/include/linux/irqdomain.h
> >> @@ -17,16 +17,14 @@
> >>   * model). It's the domain callbacks that are responsible for setting the
> >>   * irq_chip on a given irq_desc after it's been mapped.
> >>   *
> >> - * The host code and data structures are agnostic to whether or not
> >> - * we use an open firmware device-tree. We do have references to struct
> >> - * device_node in two places: in irq_find_host() to find the host matching
> >> - * a given interrupt controller node, and of course as an argument to its
> >> - * counterpart domain->ops->match() callback. However, those are treated as
> >> - * generic pointers by the core and the fact that it's actually a device-node
> >> - * pointer is purely a convention between callers and implementation. This
> >> - * code could thus be used on other architectures by replacing those two
> >> - * by some sort of arch-specific void * "token" used to identify interrupt
> >> - * controllers.
> >> + * The host code and data structures are agnostic to whether or not we
> >> + * use an open firmware device-tree. Domains can be assigned a
> >> + * "void *domain_token" identifier, which is assumed to represent a
> >> + * "struct device_node" if the pointer is a valid virtual address.
> >> + *
> >> + * Otherwise, the value is assumed to be an opaque identifier. Should
> >> + * a pointer to a non "struct device_node" be required, another data
> >> + * structure should be used to indirect it (an IDR for example).
> >>   */
> >>  
> >>  #ifndef _LINUX_IRQDOMAIN_H
> >> @@ -108,8 +106,8 @@ struct irq_domain_chip_generic;
> >>   * @flags: host per irq_domain flags
> >>   *
> >>   * Optional elements
> >> - * @of_node: Pointer to device tree nodes associated with the irq_domain. Used
> >> - *           when decoding device tree interrupt specifiers.
> >> + * @domain_token: optional firmware-specific identifier for
> >> + *                the interrupt controller
> >>   * @gc: Pointer to a list of generic chips. There is a helper function for
> >>   *      setting up one or more generic chips for interrupt controllers
> >>   *      drivers using the generic chip library which uses this pointer.
> >> @@ -130,7 +128,7 @@ struct irq_domain {
> >>  	unsigned int flags;
> >>  
> >>  	/* Optional data */
> >> -	struct device_node *of_node;
> >> +	void *domain_token;
> > 
> > I'm wondering if that may be something which isn't (void *), but a specific
> > pointer type, so the compiler warns us when something suspicious is assigned
> > to it?
> > 
> > [Somewhat along the lines struct fwnode_handle is used elsewehere.]
> 
> Yeah, I'm obviously being lazy ;-).
> 
> More seriously, I'm trying hard to avoid anything that will require an
> additional memory allocation. Going from a device_node to a
> fwnode_handle-like structure requires such an allocation which is going
> to be a massive pain to retrofit - not impossible, but painful.
> 
> What I'm currently aiming for is tagged pointers, where the two bottom
> bits indicate the type of the pointed object (see patch #3):
> - 00 indicates a device node
> - 01 indicates an IDR entry (shifted left by two bits)
> - 10 and 11 are currently unallocated, and one of them could be used to
> encode a fwnode_handle.
> 
> It would be slightly easier to replace the (void *) with a union of the
> above types:
> 
> 	union domain_token {
> 		struct device_node *of_node;
> 		struct fwnode_handle *fwnode;
> 		unsigned long id;
> 	};
> 
> That would remove the need for an extra allocation (at the cost of the
> above hack). We just need the accessors to strip the tag. Still need to
> repaint all the users of irq_domain_add_*, which is going to be
> amazingly invasive.
> 
> Thoughts?

Well, I'm not sure this is worth the effort to be honest.

I've just seen quite a few bugs where a pointer to something completely invalid
have been silently passed via (void *) which often results in very interesting
breakage (that is really hard to debug for that matter).

Thanks,
Rafael