svn commit: r360241 - head/sys/dev/ichiic

[Andriy Gapon]

On 24/04/2020 18:11, Warner Losh wrote:
> 
> 
> On Fri, Apr 24, 2020 at 8:47 AM Andriy Gapon <avg at freebsd.org
> <mailto:avg at freebsd.org>> wrote:
> 
>     On 24/04/2020 17:29, Warner Losh wrote:
>     >
>     >
>     > On Fri, Apr 24, 2020 at 1:49 AM Andriy Gapon <avg at freebsd.org
>     <mailto:avg at freebsd.org>
>     > <mailto:avg at freebsd.org <mailto:avg at freebsd.org>>> wrote:
>     >
>     >     Author: avg
>     >     Date: Fri Apr 24 07:49:21 2020
>     >     New Revision: 360241
>     >     URL: https://svnweb.freebsd.org/changeset/base/360241
>     >
>     >     Log:
>     >       ig4: ensure that drivers always attach in correct order
>     >
>     >       Use DRIVER_MODULE_ORDERED(SI_ORDER_ANY) so that ig4's ACPI attachment
>     >       happens after iicbus and acpi_iicbus drivers are registered.
>     >
>     >       I have seen a problem where iicbus attached under ig4 instead of
>     >       acpi_iicbus when ig4.ko was loaded with kldload.  I believe that that
>     >       happened because ig4 driver was a first driver to register, it attached
>     >       and created an iicbus child.  Then iicbus driver was registered and,
>     >       since it was the only driver that could attach to the iicbus child
>     >       device, it did exactly that.  After that acpi_iicbus driver was
>     >       registered.  It would be able to attach to the iicbus device, but it was
>     >       already attached, so nothing happened.
>     >
>     >
>     > Can you post more details of which devices are affected? From the description
>     > and the patch, I don't see how this could fix things.
> 
>     I think I listed them all: ig4iic with acpi attachment, iicbus and acpi_iicbus.
>     acpi
>      \--- ig4iic
>            \---- iicbus vs acpi_iicbus
> 
>     I tried to explain the problem and the fix in the commit message.  If you want
>     to discuss any specifics, let's continue with specifics.  If there is anything
>     unclear in my explanation, I can clarify, but I need to understand what needs to
>     be clarified.
> 
> 
> That won't fix the stated problem.

It will.  It does.  You made me write an essay to explain why :)

> If changing the module order fixes something,
> it's the wrong fix. Which is why I asked to make sure I understood the issue (it
> was unclear if it was at the level indicated, or for the children of the iicbus).
> 
> iicbus returns BUS_PROBE_GENERIC (-100) from its probe routine, while
> acpi_iicbus returns BUS_PROBE_DEFAULT (-20). This means that acpi_iicbus should
> always win. So something else is going on. We don't specify order on other
> devices except for some weird, special needs drivers (this is not such a driver).

This driver, along with all of other I2C controller drivers, has a particular
quirk, so it can be called weird.

It does not matter what the probe routines return if one of the drivers is not
added to "newbus" at all (even if its code is loaded).  Its probe method is not
executed.  And that's what I tried to explain in the commit message.

Now, why this driver as well as all SMBus and I2C controller drivers are
somewhat weird...  There is a multitude of drivers for SMBus and I2C
controllers.  Those drivers have different names.  So, using newbus speak, smbus
and iicbus drivers can attach to [newbus] devices under many different [newbus]
buses.  For that reason, the corresponding DRIVER_MODULE declarations are not
consolidated in smbus.c and iicbus.c; instead, they are spread over individual
controller drivers.  So, loading of smbus or iicbus module does not result in a
call to devclass_add_driver() that would register these drivers under some bus.
And that's an unusual thing comparing to the most straightforward drivers.

Let's use ig4 as an example.
Across its source files we had the following DRIVER_MODULE declarations:
DRIVER_MODULE(ig4iic, acpi, ... -- in ig4_acpi.c
DRIVER_MODULE(iicbus, ig4iic, ... -- in ig4_iic.c
DRIVER_MODULE(acpi_iicbus, ig4iic, ... -- in ig4_iic.c
The first one is needed to register ig4iic driver under acpi bus.  Other two are
needed to register iicbus and acpi_iicbus drivers under ig4iic bus.
The order is not explicitly defined, so the corresponding declaration can be
processed in any order when ig4.ko is loaded and so the corresponding
devclass_add_driver() can be called in any order.

So, I observed in practice the scenario where the drivers were added in the
written order.  First, ig4iic was added under acpi, it found the corresponding
device, probed and attached to it.  In its attach method it added a new device
named "iicbus" as a child.  Then, iicbus driver was added under ig4iic bus.
That driver found the child device and attached to it.  Only then acpi_iicbus
was added and it was too late to the party.

So, this is really about an order in which DRIVER_MODULE-s (which translate to
sysinit-s) _within a kld_ are processed.  Essentially, about an order of objects
in the corresponding section of a loadable ELF object.  MODULE_DEPEND does not
affect that order.

Just as an aside, for a contrast, gpiobus uses a different model.
There, all controller drivers must have "gpio" as their driver names (driver->name).
So, a single DRIVER_MODULE(gpiobus, gpio, ...) in the gpiobus.c code is
sufficient for gpiobus driver to be added under all controllers.  And that
registration always happens before a controller driver is added because of
MODULE_DEPEND between it and gpiobus.

> Unless I'm mistaken, the real problem is that the following line is missing instead.
> MODULE_DEPEND(ig4iic, acpi_iicbus, 1, 1, 1);
> which makes the module dependencies explicit. Can you add that to ig4_acpi.c,
> revert your change and see if that works? It will ensure that the acpi_iicbus
> driver is loaded first. If things break because it isn't, it sounds like a hard
> dependency for ig4. Since we're already pulling in acpi, that doesn't seem
> unreasonable to me.

Just for clarity, acpi_iicbus is compiled into iicbus.ko (on relevant platforms,
of course), same as iicbus.  It's not a separate kld.

-- 
Andriy Gapon