Re: Tweak I2C SDA hold time on GemniLake to make touchpad work

[Jarkko Nikula]

On 9/4/19 7:38 AM, Chris Chiu wrote:
> On Tue, Sep 3, 2019 at 8:03 PM Jarkko Nikula
> <jarkko.nikula@xxxxxxxxxxxxxxx> wrote:
> > Hi Chris
> >
> > On 9/3/19 11:18 AM, Mika Westerberg wrote:
> > > +Jarkko
> > >
> > > On Tue, Sep 03, 2019 at 04:10:27PM +0800, Chris Chiu wrote:
> > > > Hi,
> > > >
> > > > We're working on the acer Gemnilake laptop TravelMate B118-M for
> > > > touchpad not working issue. The touchpad fails to bring up and the
> > > > i2c-hid ouput the message as follows
> > > >       [    8.317293] i2c_hid i2c-ELAN0502:00: hid_descr_cmd failed
> > > > We tried on latest linux kernel 5.3.0-rc6 and it reports the same.
> > > >
> > > > We then look into I2C signal level measurement to find out why.
> > > > The following is the signal output from LA for the SCL/SDA.
> > > > https://imgur.com/sKcpvdo
> > > > The SCL frequency is ~400kHz from the SCL period, but the SDA
> > > > transition is quite weird. Per the I2C spec, the data on the SDA line
> > > > must be stable during the high period of the clock. The HIGH or LOW
> > > > state of the data line can only change when the clock signal on the
> > > > SCL line is LOW. The SDA period span across 2 SCL high, I think
> > > > that's the reason why the I2C read the wrong data and fail to initialize.
> > > >
> > > > Thus, we treak the SDA hold time by the following modification.
> > > >
> > > > --- a/drivers/mfd/intel-lpss-pci.c
> > > > +++ b/drivers/mfd/intel-lpss-pci.c
> > > > @@ -97,7 +97,8 @@ static const struct intel_lpss_platform_info bxt_uart_info = {
> > > >    };
> > > >
> > > >    static struct property_entry bxt_i2c_properties[] = {
> > > > -       PROPERTY_ENTRY_U32("i2c-sda-hold-time-ns", 42),
> > > > +       PROPERTY_ENTRY_U32("i2c-sda-hold-time-ns", 230),
> > > >           PROPERTY_ENTRY_U32("i2c-sda-falling-time-ns", 171),
> > > >           PROPERTY_ENTRY_U32("i2c-scl-falling-time-ns", 208),
> > > >           { },
> > > >
> > > > The reason why I choose sda hold time is by the Table 10 of
> > > > https://www.nxp.com/docs/en/user-guide/UM10204.pdf, the device
> > > > must provide a hold time at lease 300ns and and 42 here is relatively
> > > > too small. The signal measurement result for the same pin on Windows
> > > > is as follows.
> > > > https://imgur.com/BtKUIZB
> > > > Comparing to the same result running Linux
> > > > https://imgur.com/N4fPTYN
> > > >
> > > > After applying the sda hold time tweak patch above, the touchpad can
> > > > be correctly initialized and work. The LA signal is shown as down below.
> > > > https://imgur.com/B3PmnIp
> > Could you try does attached patch work for you?
> >
> > It's from last year for another related issue but there platform was
> > actually Apollo Lake instead of Gemini Lake but anyway it was found out
> > that Windows uses different timing parameters than Linux on Gemini Lake.
> >
> > I didn't take patch forward back then due known Gemini Lake machines
> > were working with the Broxton I2C timing parameters but now it's time if
> > attached patch fixes the issue on your machine.
> >
> > Patch is from top of v5.3-rc7 but should probably apply also to older
> > kernels.
> >
> > --
> > Jarkko
>
> Thanks, Jarkko, the patche works on my acer laptops.
Thanks. I'll send the patch out with Cc'ing you. I took the freedom to 
add your Tested-by tag if you don't mind :-)

--
Jarkko