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From:   Furquan Shaikh <furquan@google.com>
Date:   Mon, 30 Nov 2020 23:15:37 -0800
Message-ID: <CAEGmHFFuJHNpXOjzmBZ0Sjgsz-x19QFdSuns2v_uMFQyPQis=g@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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On Mon, Nov 30, 2020 at 11:06 PM Dmitry Torokhov
<dmitry.torokhov@gmail.com> wrote:
>
> On Mon, Nov 30, 2020 at 10:54:46PM -0800, Furquan Shaikh wrote:
> > 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.
>
> OK, I see.
>
> >
> > >
> > > 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.
>
> So my understanding is that not all I2C adapters support I2C_M_NOSTART
> so that is why regmap is nice as it hides it all away and figures things
> on its own.
>
> So simple solution of I2C_M_NOSTART might be a quick fix for Chrome OS
> kernel, but we'd either need to always use more expensive 2nd buffer as
> is in your patch, or regmap.

Ah I see. That makes sense. In that case, I think switching to regmap
would be better. As you suggested, I can use I2C_M_NOSTART as a quick
fix and work on enabling regmap.

>
> Thanks.
>
> --
> Dmitry