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

[Franklin S Cooper Jr.]

On 10/15/2015 07:16 PM, Dmitry Torokhov wrote:
> 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@xxxxxx wrote:
>>>> From: Franklin S Cooper Jr <fcooper@xxxxxx>
>>>>
>>>> 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@xxxxxx>
>>>> ---
>>>>  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.
Yeah that shouldn't be a problem. I'll fix that.
>
>> 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.
Sorry your right the +1 isn't needed.
>
> Can we unify the calculation to be:
>
> 	datalen = tplen * MAX_SUPPORT_POINTS + offset + crc_len;
Why do we need the crc_len? M06 is the only one that uses the CRC
and the offset insures we are reading the necessary crc registers.

Unless I'm missing something it would simply be:

datalen = tplen * MAX_SUPPORT_POINTS + offset

>
> 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.
>

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