linux-kernel - Re: [RFC 1/4] Input: edt-ft5x06 - Use max support points to determine how much to read

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Message-ID: <20151016001632.GB1794@dtor-ws>
Date:	Thu, 15 Oct 2015 17:16:32 -0700
From:	Dmitry Torokhov <dmitry.torokhov@...il.com>
To:	"Franklin S Cooper Jr." <fcooper@...com>
Cc:	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
	maxime.ripard@...e-electrons.com, LW@...O-electronics.de,
	jg1.han@...sung.com, asaf.vertz@...demg.com
Subject: Re: [RFC 1/4] Input: edt-ft5x06 - Use max support points to
 determine how much to read

On Wed, Oct 14, 2015 at 08:58:32PM -0500, Franklin S Cooper Jr. wrote:
> 
> 
> On 10/14/2015 06:39 PM, Dmitry Torokhov wrote:
> > On Wed, Oct 07, 2015 at 07:21:38AM -0500, fcooper@...com wrote:
> >> From: Franklin S Cooper Jr <fcooper@...com>
> >>
> >> Calculate the amount of data that needs to be read for the specified max
> >> number of support points. If the maximum number of support points changes
> >> then the amount that is read from the touch screen controller should
> >> reflect this.
> >>
> >> Signed-off-by: Franklin S Cooper Jr <fcooper@...com>
> >> ---
> >>  drivers/input/touchscreen/edt-ft5x06.c | 6 ++++--
> >>  1 file changed, 4 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/drivers/input/touchscreen/edt-ft5x06.c b/drivers/input/touchscreen/edt-ft5x06.c
> >> index 7239c31..1e0ed6e 100644
> >> --- a/drivers/input/touchscreen/edt-ft5x06.c
> >> +++ b/drivers/input/touchscreen/edt-ft5x06.c
> >> @@ -178,14 +178,16 @@ static irqreturn_t edt_ft5x06_ts_isr(int irq, void *dev_id)
> >>  		cmd = 0xf9; /* tell the controller to send touch data */
> >>  		offset = 5; /* where the actual touch data starts */
> >>  		tplen = 4;  /* data comes in so called frames */
> >> -		datalen = 26; /* how much bytes to listen for */
> >> +
> >> +		/* how many bytes to listen for */
> >> +		datalen = tplen * MAX_SUPPORT_POINTS + offset + 1;
> >>  		break;
> >>  
> >>  	case M09:
> >>  		cmd = 0x02;
> >>  		offset = 1;
> >>  		tplen = 6;
> >> -		datalen = 29;
> >> +		datalen = tplen * MAX_SUPPORT_POINTS - cmd + 1;
> >>  		break;
> >
> > Hmm, why would formulae for datalen be different depending on the
> > firmware? And I think original 29 it too low: we need 30 bytes for 5
> > contacts + 1 to account for offset.
> So based on the current ISR we don't care about the touch weight and
> which are the last two registers for each touch point. So for the last
> touchpoint we really don't need to read the extra two registers (-2).

This is really not obvious. I do not think we'd see any performance
degradation if we actually read the whole last touchpoint.

> We need +1 simply for the fact that we read the register at location
> cmd.

I am not sure I follow this. We do not reference anything past
rdbuf[(MAX_SUPPORT_POINTS - 1) * tplen + offset] and
our offset takes care of the start position, so why exactly we need the
+1? Ah, CRC is in the extra byte.

Can we unify the calculation to be:

	datalen = tplen * MAX_SUPPORT_POINTS + offset + crc_len;

By the way, what version of firmware you tested your changes with?

> 
> So 6 * 5 - 2 + 1 which is how we get to 29. The formula looks slightly
> different because the registers we are reading are very close to zero
> so the math works out to equal the equation I used for M09.
> 
> M06 since tplen = 4 then all four registers are used in the ISR per touch
> point. Plus the offset and plus 1 again to account for the fact we are reading
> the cmd register. But once again it would be nice if someone can confirm this.
> >
> > I also wonder why we need extra 1 byte in M06 case.
> >
> > Lothar?
> >
> > Thanks.
> >
> 

Thanks.

-- 
Dmitry
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