On Tue, 09 Jun 2020, Andy Shevchenko wrote:
> On Tue, Jun 9, 2020 at 2:01 PM Lee Jones <lee.jones@xxxxxxxxxx> wrote:
> >
> > Good morning,
> >
> > After a number of reports/queries surrounding a known long-term issue
> > in the MFD core, including the submission of a couple of attempted
> > solutions, I've decided to finally tackle this one myself.
> >
> > Currently, when a child platform device (sometimes referred to as a
> > sub-device) is registered via the Multi-Functional Device (MFD) API,
> > the framework attempts to match the newly registered platform device
> > with its associated Device Tree (OF) node. Until now, the device has
> > been allocated the first node found with an identical OF compatible
> > string. Unfortunately, if there are, say for example '3' devices
> > which are to be handled by the same driver and therefore have the same
> > compatible string, each of them will be allocated a pointer to the
> > *first* node.
> >
> > Let me give you an example.
> >
> > I have knocked up an example 'parent' and 'child' device driver. The
> > parent utilises the MFD API to register 3 identical children, each
> > controlled by the same driver. This happens a lot. Fortunately, in
> > the majority of cases, the OF nodes are also totally identical, but
> > what if you wish to configure one of the child devices with different
> > attributes or resources supplied via Device Tree, like a clock? This
> > is currently impossible.
> >
> > Here is the Device Tree representation for the 1 parent and the 3
> > child (sub) devices described above:
> >
> > parent {
> > compatible = "mfd,of-test-parent";
> >
> > child@0 {
> > compatible = "mfd,of-test-child";
> > clocks = <&clock 0>;
> > };
> >
> > child@1 {
> > compatible = "mfd,of-test-child";
> > clocks = <&clock 1>;
> > };
> >
> > child@2 {
> > compatible = "mfd,of-test-child";
> > clocks = <&clock 2>;
> > };
> > };
> >
> > This is how we register those devices from MFD:
> >
> > static const struct mfd_cell mfd_of_test_cell[] = {
> > OF_MFD_CELL("mfd_of_test_child", NULL, NULL, 0, 0, "mfd,of-test-child"),
> > OF_MFD_CELL("mfd_of_test_child", NULL, NULL, 0, 1, "mfd,of-test-child"),
> > OF_MFD_CELL("mfd_of_test_child", NULL, NULL, 0, 2, "mfd,of-test-child")
> > };
> >
> > ... which we pass into mfd_add_devices() for processing.
> >
> > In an ideal world. The devices with the platform_id; 0, 1 and 2 would
> > be matched up to Device Tree nodes; child@0, child@1 and child@2
> > respectively. Instead all 3 devices will be allocated a pointer to
> > child@0's OF node, which is obviously not correct.
> >
> > This is how it looks when each of the child devices are probed:
> >
> > [0.708287] mfd-of-test-parent mfd_of_test: Registering 3 devices
> > [...]
> > [0.712511] mfd-of-test-child mfd_of_test_child.0: Probing platform device: 0
> > [0.712710] mfd-of-test-child mfd_of_test_child.0: Using OF node: child@0
> > [0.713033] mfd-of-test-child mfd_of_test_child.1: Probing platform device: 1
> > [0.713381] mfd-of-test-child mfd_of_test_child.1: Using OF node: child@0
> > [0.713691] mfd-of-test-child mfd_of_test_child.2: Probing platform device: 2
> > [0.713889] mfd-of-test-child mfd_of_test_child.2: Using OF node: child@0
> >
> > "Why is it when I change child 2's clock rate, it also changes 0's?"
> >
> > Whoops!
> >
> > So in order to fix this, we need to make MFD more-cleverer!
> >
> > However, this is not so simple. There are some rules we should abide
> > by (I use "should" intentionally here, as something might just have to
> > give):
> >
> > a) Since Device Tree is designed to describe hardware, inserting
> > arbitrary properties into DT is forbidden. This precludes things
> > we would ordinarily be able to match on, like 'id' or 'name'.
> > b) As an extension to a) DTs should also be OS agnostic, so
> > properties like 'mfd-device', 'mfd-order' etc are also not
> > not suitable for inclusion.
> > c) The final solution should ideally be capable of supporting both
> > newly defined and current trees (without retroactive edits)
> > alike.
> > d) Existing properties could be used, but not abused. For example,
> > one of my suggestions (see below) is to use the 'reg' property.
> > This is fine in principle but loading 'reg' with arbitrary values
> > (such as; 0, 1, 2 ... x) which 1) clearly do not have anything to
> > do with registers and 2) would be meaningless in other OSes/
> > implementations, just to serve our purpose, is to be interpreted
> > as an abuse.
> >
> > Proposal 1:
> >
> > As mentioned above, my initial thoughts were to use the 'reg' property
> > to match an MFD cell entry with the correct DT node. However, not
> > all Device Tree nodes have 'reg' properties. Particularly true in the
> > case of MFD, where memory resources are usually shared with the parent
> > via Regmap, or (as in the case of the ab8500) the MFD handles all
> > register transactions via its own API.
> >
> > Proposal 2:
> >
> > If we can't guarantee that all DT nodes will have at least one
> > property in common to be used for matching and we're prevented from
> > supplying additional, potentially bespoke properties, then we must
> > seek an alternative procedure.
> >
> > It should be possible to match based on order. However, the developer
> > would have to guarantee that the order in which the child devices are
> > presented to the MFD API are in exactly the same order as they are
> > represented in the Device Tree. The obvious draw-back to this
> > strategy is that it's potentially very fragile.
> >
> > Current Proposal:
> >
> > How about a collection of Proposal 1 and Proposal 2? First we could
> > attempt a match on the 'reg' property. Then, if that fails, we would
> > use the fragile-but-its-all-we-have Proposal 2 as the fall-back.
> >
> > Thoughts?
>
> Just a side note, have you considered software nodes on the picture?
> You can add properties or additional references to the existing
> (firmware) nodes.
Is that the properties framework you are trying to replace?
Is that different to placing additional attributes into pdata?
Using my clock example above, how would one allocate a DT based clock
to a child device using properties?
--
Lee Jones [李琼斯]
Senior Technical Lead - Developer Services
Linaro.org │ Open source software for Arm SoCs
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