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SubjectRe: Tweak I2C SDA hold time on GemniLake to make touchpad work
+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
>
> The chart which has yellow mark is the tweak version, the red marks
> the original version.
>
> I need suggestions about whether if the hold time is the value I can tweak?
> Or I should modify sda falling time? Please help if any better idea.
>
> Thanks

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