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

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Date:   Tue, 15 Sep 2020 00:33:18 +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

14.09.2020 22:36, Dmitry Torokhov пишет:
> On Mon, Sep 14, 2020 at 12:33:40PM -0700, Dmitry Torokhov wrote:
>> 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@...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
>>
>> 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.

The WAKE-GPIO was never supported and I'm not sure whether anyone
actually needs it. I think we could ignore this case until anyone would
really need and could test it.

> By the way, I would like to avoid the unnecessary retries in probe paths
> if possible. I.e. on Chrome OS we really keep an eye on boot times and
> in case of multi-sourced touchscreens we may legitimately not have
> device at given address.

We could add a new MXT1386 DT compatible and then do:

static void mxt_wake(struct mxt_data *data)
{
	struct i2c_client *client = data->client;
	struct device *dev = &data->client->dev;
	union i2c_smbus_data dummy;

	if (!of_device_is_compatible(dev, "atmel,mXT1386"))
		return;

	/* TODO: add WAKE-GPIO support */

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

	msleep(MXT_WAKEUP_TIME);
}

Jiada, will you be able to re-work this patch? Please note that the new
"atmel,mXT1386" DT compatible needs to be added into the
atmel,maxtouch.txt binding.