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From:   Furquan Shaikh <furquan@google.com>
Date:   Mon, 30 Nov 2020 22:54:46 -0800
Message-ID: <CAEGmHFGYuM91U+Tvq+YDt180cfHQKnOKMY5ToHZ7v44fOs-_8w@mail.gmail.com>
Subject: Re: [PATCH] input: raydium_ts_i2c: Do not split tx transactions
To:     Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc:     Dan Carpenter <dan.carpenter@oracle.com>,
        linux-input@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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Hello Dmitry,

On Mon, Nov 30, 2020 at 10:28 PM Dmitry Torokhov
<dmitry.torokhov@gmail.com> wrote:
>
> Hi Furquan,
>
> On Mon, Nov 30, 2020 at 10:00:50PM -0800, Furquan Shaikh wrote:
> > Raydium device does not like splitting of tx transactions into
> > multiple messages - one for the register address and one for the
> > actual data. This results in incorrect behavior on the device side.
> >
> > This change updates raydium_i2c_read and raydium_i2c_write to create
> > i2c_msg arrays separately and passes those arrays into
> > raydium_i2c_xfer which decides based on the address whether the bank
> > switch command should be sent. The bank switch header is still added
> > by raydium_i2c_read and raydium_i2c_write to ensure that all these
> > operations are performed as part of a single I2C transfer. It
> > guarantees that no other transactions are initiated to any other
> > device on the same bus after the bank switch command is sent.
>
> i2c_transfer locks the bus [segment] for the entire time, so this
> explanation on why the change is needed does not make sense.

The actual problem is with raydium_i2c_write chopping off the write
data into 2 messages -- one for register address and other for actual
data. Raydium devices do not like that. Hence, this change to ensure
that the register address and actual data are packaged into a single
message. The latter part of the above comment attempts to explain why
the bank switch message is added to xfer[] array in raydium_i2c_read
and raydium_i2c_write instead of sending a separate message in
raydium_i2c_xfer i.e. to ensure that the read/write xfer and bank
switch are sent to i2c_transfer as a single array of messages so that
they can be handled as an atomic operation from the perspective of
communication with this device on the bus.

>
> Also, does it help if you mark the data message as I2C_M_NOSTART in case
> of writes?

That is a great suggestion. I think this would be helpful in this
scenario. Let me follow-up on this to see if it helps with the current
problem.

>
> I also wonder if we should convert the driver to regmap, which should
> help with handling the bank switch as well as figuring out if it can do
> "gather write" or fall back to allocating an additional send buffer.

I will start with the above suggestion and fallback to this if that
doesn't work.

Thanks for the quick response and the helpful suggestions Dmitry. I
will work on these pointers and get back to you. Thanks again.

- Furquan

>
> Thanks.
>
> --
> Dmitry