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

Dmitry Torokhov
2020-09-14T22:32:47Z

On Tue, Sep 15, 2020 at 12:33:18AM +0300, Dmitry Osipenko wrote:
> 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@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.
>
> 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.

Right, I am not advocating adding GPIO control right away, I was simply
arguing why I believe we should separate wakeup handling from normal
communication handling.

>
> > 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.

Another option is to have a device property "atmel,wakeup-type" or
something, in case there are more Atmel variants needing this.

Rob, any preferences here?

Thanks.

--
Dmitry