Detecting if a laptop is using a i2c/smbus or a ps/2 attached touchpad?

Detecting if a laptop is using a i2c/smbus or a ps/2 attached touchpad?

[Hans de Goede]

Hi Benjamin, et. al.,

I have just merged a patch which adds a couple of DMI quirks to disable
the touchpad on/off functionality of the ideapad-laptop module on some models,
because it is causing issues there.

Patch:      https://lore.kernel.org/platform-driver-x86/20221018095323.14591-1-limanyi@uniontech.com/
Discussion: https://lore.kernel.org/platform-driver-x86/fdceb6d2-c38c-e8ff-4c19-b01521ca46c0@redhat.com/

Rather then using DMI quirks I would prefer to just disable it everywhere
where a i2c-attached touchpad is used. Be it either one only attached to
i2c, or one of the synaptics intertouch thingies. I'm still trying to
figure out why the ideapad-laptop module's touchpad control (which
uses the EC + enabling/disabling the i8042 aux-port) matters for
i2c-attached touchpads at all, so I suspect that intertouch might be
a part of it.

So now I'm wondering if there is some nice way for the ideapad-laptop
code to check if the touchpad is intertouch and/or if it is i2c-attached
only ...

Any ideas?

Regards,

Hans

p.s.

Is it possible that Elan touchpads use something similar to intertouch,
or is that a synaptics thing only (under Linux)  ?

Re: Detecting if a laptop is using a i2c/smbus or a ps/2 attached touchpad?

[Benjamin Tissoires]

Hi Hans,

On Mon, Nov 7, 2022 at 11:10 AM Hans de Goede <hdegoede@redhat.com> wrote:
>
> Hi Benjamin, et. al.,
>
> I have just merged a patch which adds a couple of DMI quirks to disable
> the touchpad on/off functionality of the ideapad-laptop module on some models,
> because it is causing issues there.
>
> Patch:      https://lore.kernel.org/platform-driver-x86/20221018095323.14591-1-limanyi@uniontech.com/
> Discussion: https://lore.kernel.org/platform-driver-x86/fdceb6d2-c38c-e8ff-4c19-b01521ca46c0@redhat.com/
>
> Rather then using DMI quirks I would prefer to just disable it everywhere
> where a i2c-attached touchpad is used. Be it either one only attached to
> i2c, or one of the synaptics intertouch thingies. I'm still trying to
> figure out why the ideapad-laptop module's touchpad control (which
> uses the EC + enabling/disabling the i8042 aux-port) matters for
> i2c-attached touchpads at all, so I suspect that intertouch might be
> a part of it.
>
> So now I'm wondering if there is some nice way for the ideapad-laptop
> code to check if the touchpad is intertouch and/or if it is i2c-attached
> only ...
>
> Any ideas?

TL;DR: it's a complete mess.

For mostly UEFI/BIOS compatibility, all touchpads on modern laptops
have a PS/2 interface.

But then, most of the touchpads (and of course not all) can also use
I2C or SMBus to communicate.

And this is where things are bad:

- when using i2c-hid, there is a DT entry that enumerates the
touchpad, but sometimes the manufacturers like to reuse the same DT
and just enable/disable devices in it, so it's not easy to do.
In the i2c-hid case, the PS/2 node has no idea it is not used anymore,
and thus is kept up forever

- for touchpads using SMBus (on Intel platforms only currently), the
enumeration is done at the PS/2 level: there is a byte that says that
the touchpad supports another bus or not.
This is valid for both Synaptics, and Elan, as you figured out.

But in the SMBus case there are many cases where it is not working: if
the SMBus doesn't support Host Notify (AMD platforms), or if the
enumeration is not working because the touchpad is not happy, etc...

So again, in SMBus, we can not disable the PS/2 nodes, because there
is a high chance the touchpad will not work. Not to mention that the
specific bit to enable the touchpad might also be required when coming
back from suspend (not entirely sure though).

However, in the Synaptics SMbus case, we took a safer approach
(because the SMBus driver is not standard and sometimes breaks on new
hardware), so we are using the PnP ID to selectively enable the
devices (see smbus_pnp_ids[] in drivers/input/mouse/synaptics.c)

In the Elan case, if the touchpad exports the magic bit, we forward it
to SMBus as the driver is behaving better with new devices.

As a result, from user space, it's easier to know if the device uses
smbus/i2c, simply because you can enumerate all devices.
But from the kernel, either you have to look for the ACPI tables, and
hope that the I2C bus is correctly supported in the system, or you
have to talk over PS/2 and hope that the platform supports
HostNotify...

>
> Regards,
>
> Hans
>
> p.s.
>
> Is it possible that Elan touchpads use something similar to intertouch,
> or is that a synaptics thing only (under Linux)  ?
>

Yep, see above. If the touchpad says it can be driven through SMBus,
we forward it.

Cheers,
Benjamin

Re: Detecting if a laptop is using a i2c/smbus or a ps/2 attached touchpad?

[Hans de Goede]

Hi,

On 11/7/22 11:52, Benjamin Tissoires wrote:
> Hi Hans,
> 
> On Mon, Nov 7, 2022 at 11:10 AM Hans de Goede <hdegoede@redhat.com> wrote:
>>
>> Hi Benjamin, et. al.,
>>
>> I have just merged a patch which adds a couple of DMI quirks to disable
>> the touchpad on/off functionality of the ideapad-laptop module on some models,
>> because it is causing issues there.
>>
>> Patch:      https://lore.kernel.org/platform-driver-x86/20221018095323.14591-1-limanyi@uniontech.com/
>> Discussion: https://lore.kernel.org/platform-driver-x86/fdceb6d2-c38c-e8ff-4c19-b01521ca46c0@redhat.com/
>>
>> Rather then using DMI quirks I would prefer to just disable it everywhere
>> where a i2c-attached touchpad is used. Be it either one only attached to
>> i2c, or one of the synaptics intertouch thingies. I'm still trying to
>> figure out why the ideapad-laptop module's touchpad control (which
>> uses the EC + enabling/disabling the i8042 aux-port) matters for
>> i2c-attached touchpads at all, so I suspect that intertouch might be
>> a part of it.
>>
>> So now I'm wondering if there is some nice way for the ideapad-laptop
>> code to check if the touchpad is intertouch and/or if it is i2c-attached
>> only ...
>>
>> Any ideas?
> 
> TL;DR: it's a complete mess.
> 
> For mostly UEFI/BIOS compatibility, all touchpads on modern laptops
> have a PS/2 interface.
> 
> But then, most of the touchpads (and of course not all) can also use
> I2C or SMBus to communicate.
> 
> And this is where things are bad:
> 
> - when using i2c-hid, there is a DT entry that enumerates the
> touchpad, but sometimes the manufacturers like to reuse the same DT
> and just enable/disable devices in it, so it's not easy to do.
> In the i2c-hid case, the PS/2 node has no idea it is not used anymore,
> and thus is kept up forever
> 
> - for touchpads using SMBus (on Intel platforms only currently), the
> enumeration is done at the PS/2 level: there is a byte that says that
> the touchpad supports another bus or not.
> This is valid for both Synaptics, and Elan, as you figured out.
> 
> But in the SMBus case there are many cases where it is not working: if
> the SMBus doesn't support Host Notify (AMD platforms), or if the
> enumeration is not working because the touchpad is not happy, etc...
> 
> So again, in SMBus, we can not disable the PS/2 nodes, because there
> is a high chance the touchpad will not work. Not to mention that the
> specific bit to enable the touchpad might also be required when coming
> back from suspend (not entirely sure though).
> 
> However, in the Synaptics SMbus case, we took a safer approach
> (because the SMBus driver is not standard and sometimes breaks on new
> hardware), so we are using the PnP ID to selectively enable the
> devices (see smbus_pnp_ids[] in drivers/input/mouse/synaptics.c)
> 
> In the Elan case, if the touchpad exports the magic bit, we forward it
> to SMBus as the driver is behaving better with new devices.
> 
> As a result, from user space, it's easier to know if the device uses
> smbus/i2c, simply because you can enumerate all devices.
> But from the kernel, either you have to look for the ACPI tables, and
> hope that the I2C bus is correctly supported in the system, or you
> have to talk over PS/2 and hope that the platform supports
> HostNotify...
> 
>>
>> Regards,
>>
>> Hans
>>
>> p.s.
>>
>> Is it possible that Elan touchpads use something similar to intertouch,
>> or is that a synaptics thing only (under Linux)  ?
>>
> 
> Yep, see above. If the touchpad says it can be driven through SMBus,
> we forward it.

Ok, thank you for the long answer.

I will try to find some generic solution on the ideapad-laptop driver
side. First of all we need to root cause which call in that driver
is exactly causing the touchpad to not work.

Regards,

hans