Re: [PATCH v4 1/1] Input: atmel_mxt_ts - implement I2C retries

Dmitry Torokhov
2020-09-14T19:33:50Z

On Mon, Sep 14, 2020 at 08:29:44PM +0300, Dmitry Osipenko wrote:
> 13.09.2020 19:56, Dmitry Torokhov пишет:
> > Hi Jiada,
> >
> > On Sat, Sep 12, 2020 at 09:55:21AM +0900, Jiada Wang wrote:
> >> From: Nick Dyer <nick.dyer@itdev.co.uk>
> >>
> >> Some maXTouch chips (eg mXT1386) will not respond on the first I2C request
> >> when they are in a sleep state. It must be retried after a delay for the
> >> chip to wake up.
> >
> > Do we know when the chip is in sleep state? Can we do a quick I2C
> > transaction in this case instead of adding retry logic to everything? Or
> > there is another benefit for having such retry logic?
>
> Hello!
>
> Please take a look at page 29 of:
>
> https://ww1.microchip.com/downloads/en/DeviceDoc/mXT1386_1vx_Datasheet_LX.pdf
>
> It says that the retry is needed after waking up from a deep-sleep mode.
>
> There are at least two examples when it's needed:
>
> 1. Driver probe. Controller could be in a deep-sleep mode at the probe
> time, and then first __mxt_read_reg() returns I2C NACK on reading out TS
> hardware info.
>
> 2. Touchscreen input device is opened. The touchscreen is in a
> deep-sleep mode at the time when input device is opened, hence first
> __mxt_write_reg() invoked from mxt_start() returns I2C NACK.
>
> I think placing the retries within __mxt_read() / write_reg() should be
> the most universal option.
>
> Perhaps it should be possible to add mxt_wake() that will read out some
> generic register

I do not think we need to read a particular register, just doing a quick
read:

i2c_smbus_xfer(client->adapter, client->addr,
0, I2C_SMBUS_READ, 0, I2C_SMBUS_BYTE, &dummy)

should suffice.

> and then this helper should be invoked after HW
> resetting (before mxt_read_info_block()) and from mxt_start() (before
> mxt_set_t7_power_cfg()). But this approach feels a bit fragile to me.
>

Actually, reading the spec, it all depends on how the WAKE pin is wired
up on a given board. In certain setups retrying transaction is the right
approach, while in others explicit control is needed. So indeed, we need
a "wake" helper that we should call in probe and resume paths.

Thanks.

--
Dmitry