Re: ABS_PRESSURE and 0 ranges

You wrote:

On Thursday, October 25, 2012 10:11:27 PM Rolf Eike Beer wrote:

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You wrote:

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Hi Rolf,

On Thu, Oct 25, 2012 at 02:59:49PM +0200, Rolf Eike Beer wrote:

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Hi all,

as I found out the hard way tslib does the detection if a touchscreen
has
been "clicked" if the kernel driver says it does not support
ABS_PRESSURE. But when the kernel says it can do ABS_PRESSURE tslib
will
not do it's emulation and just pass on whatever pressure value the
kernel
sends.

So far, so good. Sadly there are some drivers that do:

input_set_abs_params(dev, ABS_PRESSURE, 0, 0, 0, 0);

What drivers are doing this?

git grep 'ABS_PRESSURE, 0, 0, 0, 0' drivers/input/

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Guess what happens: no clicks at all. The question I have is: does the
above line make any sense to be in a kernel driver at all? Or is that
some voodoo that must be there and it's all tslib fault?

Now that I know where the problem is coming from the problem is easily
fixable, I'm just wondering which patch to make.

Well, the limits are strictly advisory for userspace, input core does
not perform any clamping or discarding the data that falls outside the
limits, so tslib probably should not do that either.

My main concern is that if the driver says it will only ever give pressure
values from 0 to 0, what useful stuff may that ever be? And since the
driver says it supports presure touchscreen will wait for the driver to
report it. Since it will never do it (or could at most return a 0), then
there is nothing that could ever become a valid click event.

Once again, the limits are simply advisory and can even be adjusted from
userspace (EVIOCSABS).

So ucb1400_ts does report true pressure so all is good there, but tsc40 is
not. I think the claim for ABS_PRESSURE is left over from time when tslib
did not think that device without ABS_PRESSURE was a touchscreen. I'll
remove it from the driver.

Thanks, so I know what part to patch. Please backport that to stable, too, so 
tslib will work with that driver even on stable kernels.

Eike