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

[Dmitry Torokhov <dmitry.torokhov@gmail.com>]

On Fri, Apr 28, 2017 at 08:41:56AM +0900, Andi Shyti wrote:
> 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?

It depends on what the common case is. It looks like for touch data you
always do 2 i2c transactions per interrupt. I wonder if doing it once
and paying the price of overhead for debug a nd confirmation events is
not worth it.

Thanks.

-- 
Dmitry