Re: [PATCH 01/26] Input: synaptics-rmi4 - embed the function modules in rmi_core

From: Andrew Duggan
Date: Thu Nov 12 2015 - 16:32:39 EST

On 11/10/2015 01:03 AM, Benjamin Tissoires wrote:
On Tue, Nov 10, 2015 at 12:06 AM, Dmitry Torokhov
<dmitry.torokhov@xxxxxxxxx> wrote:
On Thu, Nov 05, 2015 at 03:36:18PM -0800, Andrew Duggan wrote:
From: Benjamin Tissoires <benjamin.tissoires@xxxxxxxxxx>

the function modules can not be auto-loaded by udev. So at boot, the
functions are not there and the device is not properly populated.
Force the functions to be embedded in rmi_core so that when the touchpad
is there, the functions are there too.
There is nothing inherently different in RMI compared to other buses.
kmod package simply needs to be aware of it.

I can't help but thinking that it is slightly different though. We
register one RMI bus like the others, but then, the only driver
(rmi-driver) on the bus needs to enumerate the device and attach other
kernel modules on demand. It looks as if the rmi device is an other
internal bus. But the current implementation only allows the functions
to be loaded during the probe of the rmi_device.

During this probe, we can't block to wait for userspace to load the
various modules, and so we are screwed. The solution would be to allow
deferring the loading of the various functions, which basically comes
down to create a sub-bus per device.

The way I see it is:
- for 90 % of the cases, RMI4 will be used for touchpads in general
laptops. Distributions will likely enable 2D sensors, F30, fingerprint
readers, and maybe a few others. I don't think we want to chase all
the initrd tools to include the various RMI modules or people will
have a non functional touchpad.
- for the rest (embedded, phones, etc,...), these projects usually
already configure their own kernels and they can decide whether or not
they want to include which function depending on the actual hardware.

I am not saying that having autoloading is a bad thing. I just can't
see the interest for the general use case, and I can see the nightmare
to maintain the autoloading :).


Conceptually, I like the idea of having functions as modules. That way you can only load the function drivers which match your device. But, as Benjamin points out supporting that really complicates initialization. Especially, when individual functions add capabilities to a single input device. If loading a module gets delayed then the input device can get registered with an incomplete set of capabilities which could cause userspace to misidentify the device. It looks like the input device's capabilities will get updated after the module has loaded. But, userspace doesn't seem to get a notification that those capabilities have changed. I guess one option would be to delete and recreate the input device if the capabilities change. Unless there is a better way to generate a notification?

I've been looking into getting modules working the last couple of days and I haven't really found a good solution. I tried adding the appropriate modalias and uevent entries to the function drivers and setting up the various fields in mod_devicetable.h and file2alias.c so that udev would automatically load the modules when the device is created. I haven't gotten this method to work so I am probably missing something. But, there is no guarantee that the module will be loaded before the input device is registered.

I also tried adding a call to request_module() to load the module just before registering the function driver. It works, but there is no guarantee that the module will be ready when we do the register. Especially, if we don't want to wait and use the _nowait version of request_module.

We could change the initialization process to be more asynchronous. We could record which functions have modules (based on the return value of request_module) and then register for notifications when the function driver modules load. Only after all of the modules have loaded do we register the input device.

But, these solutions add a lot of complexity and I'm not sure it is worth it given the size and number of function drivers. Like Benjamin said, on general laptops there will be a few extra function drivers in the core. While more resource constrained systems like phones usually don't support modules and will have only the function drivers enabled in the config which exist on the device. Unless there is a better approach I haven't found, I am inclined to just go ahead and include function drivers in the core. At least for now.

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