linux-kernel - Re: [PATCH v4 1/1] Input: atmel_mxt_ts

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Message-ID: <65d1b9f2-a8e3-6398-712d-41d8067d06a1@gmail.com>
Date:   Mon, 14 Sep 2020 20:29:44 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Jiada Wang <jiada_wang@...tor.com>
Cc:     nick@...anahar.org, linux-input@...r.kernel.org,
        linux-kernel@...r.kernel.org, andy.shevchenko@...il.com,
        erosca@...adit-jv.com, Andrew_Gabbasov@...tor.com
Subject: Re: [PATCH v4 1/1] Input: atmel_mxt_ts - implement I2C retries

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@...ev.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 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.

AFAIK, there shouldn't be any extra benefits from the retrying other
than to handle the deep-sleep of mxt1386.