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Date:	Mon, 24 Feb 2014 02:18:21 +0100
From:	Michal Malý <madcatxster@...fuk.cz>
To:	Anssi Hannula <anssi.hannula@....fi>
Cc:	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
	dmitry.torokhov@...il.com, elias.vds@...il.com, jkosina@...e.cz,
	simon@...gewell.org
Subject: Re: [PATCH v2 1/4] Add ff-memless-next driver

On Monday 24 of February 2014 02:32:29 Anssi Hannula wrote:
> 24.02.2014 01:29, Michal Malý kirjoitti:
> > Introduce ff-memless-next module as a possible future replacement of
> > ff-memless.
> > 
> > Tested-by: Elias Vanderstuyft <elias.vds@...il.com>
> > Signed-off-by: Michal Malý <madcatxster@...fuk.cz>
> 
> Some comments below.
> 
> > ---
> > 
> >  v2:
> >   Handle upload and removal of uncombinable effects correctly
> >   Remove redundant information from the documentation file
> >   Invert direction of force along Y axis to conform to common conventions
> >   Set FF_GAIN bit
> >  
> >  Documentation/input/ff-memless-next.txt | 141 ++++++
> >  drivers/input/Kconfig                   |  11 +
> >  drivers/input/Makefile                  |   1 +
> >  drivers/input/ff-memless-next.c         | 789
> >  ++++++++++++++++++++++++++++++++ include/linux/input/ff-memless-next.h  
> >  |  32 ++
> >  5 files changed, 974 insertions(+)
> >  create mode 100644 Documentation/input/ff-memless-next.txt
> >  create mode 100644 drivers/input/ff-memless-next.c
> >  create mode 100644 include/linux/input/ff-memless-next.h
> > 
> > diff --git a/Documentation/input/ff-memless-next.txt
> > b/Documentation/input/ff-memless-next.txt new file mode 100644
> > index 0000000..1b550dc
> > --- /dev/null
> > +++ b/Documentation/input/ff-memless-next.txt
> > @@ -0,0 +1,141 @@
> 
> [...]
> 
> > +3. API provided by "ff-memless-next"
> > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > +"ff-memless-next" provides an API for developers of hardware-specific
> > +drivers. In order to use the API, the hardware-specific driver should
> > +include <linux/input/ff-memless-next.h>
> 
> > +Functions and structs defined by this API are:
> [...]
> 
> This kind of basic API documentation belongs in the headers (kernel-doc
> format).
> 
> [...]
> 
> > +	MLNX_UPLOAD_UNCOMB - Check if the device can accept and play
> > +			     "uncombinable" effect and upload the effect into
> > +			     the device's internal memory.
> > +			     Hardware-specific driver should return 0
> > +			     on success.
> > +	MLNX_ERASE_UNCOMB - Remove "uncombinable" effect from device's
> > +			    internal memory.
> > +			    Hardware-specific driver should return 0
> > +			    on success.
> > +	MLNX_START_UNCOMB - Start playback of "uncombinable" effect.
> > +	MLNX_STOP_UNCOMB - Stop playback of "uncombinable" effect.
> 
> These seem to be unused by any drivers?
> 
> I don't think they should be added to the kernel before there is an
> implementation using them (it also makes reviewing them more difficult).

They are used by my dummy driver (link to the source is in the documentation). 
Handling uncombinable effects in lg4ff would be a bit more involved so I 
decided to add support for these in a separate patch series.

> [...]
> 
> > +4. Specifics of "ff-memless-next" for userspace developers
> > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > +None of the persons involved in development or testing of
> > "ff-memless-next" +is aware of any reference force feedback
> > specifications. "ff-memless-next" +tries to adhere to Microsoft's
> > DirectInput specifications because we +believe that is what most
> > developers have experience with.
> > +
> > +- Waveforms of periodic effects do not start at the origin, but as
> > follows: +	SAW_UP: At minimum
> > +	SAW_DOWN: At maximum
> > +	SQUARE: At maximum
> > +	TRIANGLE: At maximum
> > +	SINE: At origin
> > +
> > +- Updating periodic effects:
> > +	- All periodic effects are restarted when their parameters are 
updated.
> > +
> > +- Updating effects of finite duration:
> > +	- Once an effect of finite length finishes playing, it is considered
> > +	  stopped. Only effects that are playing can be updated on the fly.
> > +	  Therefore effects of finite duration can be updated only until
> > +	  they finish playing.
> 
> Stopped effects should still be able to be updated.
> 
> Anyway, if you want to target userspace developers with this
> information, you should put it in generic documentation ("it is not
> guaranteed that X", etc.). If not, don't say "for userspace developers".
> 
> [...]
> 
> > diff --git a/drivers/input/ff-memless-next.c
> > b/drivers/input/ff-memless-next.c new file mode 100644
> > index 0000000..843a223
> > --- /dev/null
> > +++ b/drivers/input/ff-memless-next.c
> > @@ -0,0 +1,789 @@
> > +/*
> > + * Force feedback support for memoryless devices
> > + *
> > + * This module is based on "ff-memless" orignally written by Anssi
> > Hannula. + * It is extended to support all force feedback effects
> > currently supported + * by the Linux input stack.
> > + *
> > + * Copyright(c) 2013 Michal Maly <madcatxster@...fuk.cz>
> > + *
> > + */
> 
> [...]
> 
> > +static int mlnx_upload(struct input_dev *dev, struct ff_effect *effect,
> > +		       struct ff_effect *old)
> > +{
> 
> [...]
> 
> > +		}
> > +
> > +		mlnx_schedule_playback(mlnxdev);
> > +		spin_unlock_irq(&dev->event_lock);
> > +		return 0;
> 
> The above two lines are unneeded.
Oops...

> > +	}
> > +
> > +	spin_unlock_irq(&dev->event_lock);
> > +
> > +	return 0;
> > +}
> > +
> 
> [...]
> 
> > diff --git a/include/linux/input/ff-memless-next.h
> > b/include/linux/input/ff-memless-next.h new file mode 100644
> > index 0000000..ba89ba1
> > --- /dev/null
> > +++ b/include/linux/input/ff-memless-next.h
> > @@ -0,0 +1,32 @@
> 
> [...]
> 
> > +int input_ff_create_mlnx(struct input_dev *dev, void *data,
> > +			 int(*control_effect)(struct input_dev *, void *, const 
struct
> > mlnx_effect_command *), +			 const u16 update_rate);
> 
> Why is update_rate now driver-selectable?
Because it is my intention to replace ff-memless with MLNX, I want to account 
for (very old?) devices that might have a limit on how many commands they can 
accept within a given timeframe. Update interval in ff-memless is 50 msecs 
whereas Logitech wheels can go as low as 8 msecs. The lower the update 
interval - the better the quality of periodic, ramp and envelope effects. 
Allowing the driver to choose how fast will MLNX generate updates seemed like 
the best way to go.


Your feedback is very much appreciated...

Michal M.
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