linux-kernel - Re: [PATCH v3 2/2] Input: add support for the STMicroelectronics FingerTip touchscreen

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Date:   Fri, 28 Apr 2017 08:41:56 +0900
From:   Andi Shyti <andi.shyti@...sung.com>
To:     Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Chanwoo Choi <cw00.choi@...sung.com>,
        linux-input@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, Andi Shyti <andi@...zian.org>,
        javier@...hile0.org, Andi Shyti <andi.shyti@...sung.com>
Subject: Re: [PATCH v3 2/2] Input: add support for the STMicroelectronics
 FingerTip touchscreen

Hi Dmitry,

On Wed, Apr 26, 2017 at 05:39:18PM -0700, Dmitry Torokhov wrote:
> 
> On Mon, Mar 27, 2017 at 10:07:43PM +0900, Andi Shyti wrote:
> > +static irqreturn_t stmfts_irq_handler(int irq, void *dev)
> > +{
> > +	struct stmfts_data *sdata = dev;
> > +	int ret;
> > +
> > +	mutex_lock(&sdata->mutex);
> > +	ret = i2c_smbus_read_i2c_block_data(sdata->client,
> > +						STMFTS_READ_ONE_EVENT,
> > +						STMFTS_EVENT_SIZE, sdata->data);
> > +
> > +	if (ret < 0 || ret != STMFTS_EVENT_SIZE)
> > +		goto exit;
> 
> Why do we split read into 2 chunks? Can we issue STMFTS_READ_ALL_EVENT
> right away instead of reading first event, analyzing it, and then (maybe)
> fetching the rest?

The reason is that I don't need to read all the events at once
anytime, for example debug events or confirmation events normally
occur with a single event in the fifo. In this case I would read
only 32bytes instead of 256bytes.

Unfortunately there are no other ways to know how many events are
in the queue beforehand.

There are some "magic" commands to figure that out, but this is
specific to the Samsung's version of the stmfts and I don't want
to push it to everyone else.

The difference between this version of the driver and the
previous one is that in this one if I stress-use of the
touchscreen, the throughput is optimised (e.g. if I use more
fingers).
Before I was reading single events at time, establishing for each
read an i2c "handshake", this was de-synchronizing the protocol.

> Also, why do we use smbus protocol for the first event and i2c for the
> rest?

Standing to the datasheet, the device is smbus compatible and it
should use smbus all the time. The problem is that here the
protocol is broken in case I want to read out the full FIFO,
which has a total of 256bytes and I have to force the read by
using the function "stmfts_read_i2c_block_data()".

Personally I don't like these kind of i2c reads, because they
duplicate code, the SMBUS does that already, this is why in the
previous version I was reading the events one by one.

Do you think it is better to make a single read of all the fifo?

Andi