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:

quoted

13.09.2020 19:56, Dmitry Torokhov пишет:

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Hi Jiada,

On Sat, Sep 12, 2020 at 09:55:21AM +0900, Jiada Wang wrote:

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From: Nick Dyer <redacted>

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.

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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.

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.

Thanks.

-- 
Dmitry