Re: [PATCH v4 01/24] input: Add ff-memless-next module

[Michal Malý]

On Tuesday 20 of May 2014 11:32:14 Roland Bosa wrote:
> On 05/20/2014 02:27 AM, Michal Malý wrote:
> > On Wednesday 14 of May 2014 11:05:58 Dmitry Torokhov wrote:
> >> On Wed, May 14, 2014 at 10:35:25AM +0200, Michal Malý wrote:
> >>> Hi Dmitry,
> >>> 
> >>> thank you for reviewing this.
> >>> 
> >>> On Tuesday 13 of May 2014 23:38:06 Dmitry Torokhov wrote:
> >>>> On Sat, Apr 26, 2014 at 05:02:00PM +0200, Michal Malý wrote:
> >>>>> +
> >>>>> +/** DEFINITION OF TERMS
> >>>>> + *
> >>>>> + * Combined effect - An effect whose force is a superposition of
> >>>>> forces
> >>>>> + *                   generated by all effects that can be added
> >>>>> together.
> >>>>> + *                   Only one combined effect can be playing at a
> >>>>> time.
> >>>>> + *                   Effects that can be added together to create a
> >>>>> combined + *                   effect are FF_CONSTANT, FF_PERIODIC and
> >>>>> FF_RAMP. + * Uncombinable effect - An effect that cannot be combined
> >>>>> with
> >>>>> another effect. + *                       All conditional effects -
> >>>>> FF_DAMPER, FF_FRICTION, + *                       FF_INERTIA and
> >>>>> FF_SPRING are uncombinable. + *                       Number of
> >>>>> uncombinable effects playing simultaneously + *
> >>>>> depends on the capabilities of the hardware. + * Rumble effect - An
> >>>>> effect generated by device's rumble motors instead of + *
> >>>>> force feedback actuators.
> >>>>> + *
> >>>>> + *
> >>>>> + * HANDLING OF UNCOMBINABLE EFFECTS
> >>>>> + *
> >>>>> + * Uncombinable effects cannot be combined together into just one
> >>>>> effect,
> >>>>> at + * least not in a clear and obvious manner. Therefore these
> >>>>> effects
> >>>>> have to + * be handled individually by ff-memless-next. Handling of
> >>>>> these
> >>>>> effects is + * left entirely to the hardware-specific driver,
> >>>>> ff-memless-next merely + * passes these effects to the
> >>>>> hardware-specific
> >>>>> driver at appropriate time. + * ff-memless-next provides the UPLOAD
> >>>>> command to notify the hardware-specific + * driver that the userspace
> >>>>> is
> >>>>> about to request playback of an uncombinable + * effect. The
> >>>>> hardware-specific driver shall take all steps needed to make + * the
> >>>>> device ready to play the effect when it receives the UPLOAD command. +
> >>>>> *
> >>>>> The actual playback shall commence when START_UNCOMB command is
> >>>>> received.
> >>>>> + * Opposite to the UPLOAD command is the ERASE command which tells +
> >>>>> *
> >>>>> the hardware-specific driver that the playback has finished and that +
> >>>>> *
> >>>>> the effect will not be restarted. STOP_UNCOMB command tells
> >>>>> + * the hardware-specific driver that the playback shall stop but the
> >>>>> device + * shall still be ready to resume the playback immediately.
> >>>>> + *
> >>>>> + * In case it is not possible to make the device ready to play an
> >>>>> uncombinable + * effect (all hardware effect slots are occupied), the
> >>>>> hardware-specific + * driver may return an error when it receives an
> >>>>> UPLOAD command. If the
> >>>> 
> >>>> This part concerns me. It seems to me that devices supporting
> >>>> "uncombinable" effects are in fact not memoryless devices and we should
> >>>> not be introducing this term here. If the goal is to work around
> >>>> limited
> >>>> number of effect slots in the devices by combining certain effects then
> >>>> it needs to be done at ff-core level as it will be potentially useful
> >>>> for all devices.
> >>> 
> >>> Force generated by a conditional effect (referred to as "uncombinable"
> >>> within ff-memless-next to make the distinction clear) depends on a
> >>> position of the device. For instance the more a device is deflected from
> >>> a neutral position the greater force FF_SPRING generates. A truly
> >>> memoryless device would have to report its position to the driver, have
> >>> it calculate the appropriate force and send it back to the device. IMHO
> >>> such a loop would require a very high USB polling rate to play
> >>> conditional effects with acceptable quality.
> >>> 
> >>> We know for a fact that at least many (all?) Logitech devices that
> >>> support
> >>> conditional effects use this "semi-memoryless" approach where
> >>> FF_CONSTANT
> >>> and FF_PERIODIC are handled in the memoryless fashion and conditional
> >>> effects are uploaded to the device (in a somewhat simplified form). The
> >>> amount of effects that can be uploaded to a device is limited which is
> >>> why ff-memless-next uses two steps (UPLOAD/ERASE and START/STOP) to
> >>> handle these effects.
> >>> 
> >>> Conditional effects - even if they are of the same type - cannot be
> >>> effectively combined into one because superposition doesn't seem to work
> >>> here so they have to be processed one by one.
> >>> 
> >>> If we ever come across a really memoryless device it should not be
> >>> particularly difficult to add another callback to ff-memless-next which
> >>> would emulate conditional effects with constant force.
> >> 
> >> Thank you for the explanation. This further solidifies for me the idea
> >> that handling of such effects that are in fact uploaded to and managed
> >> by the device should not be handled by the memoryless core but rather by
> >> the driver itself. I.e. such drivers should implement their own play(),
> >> upload(), erase(), etc, and decide whether to use a hardware slot for
> >> the effect or handle effect in memoryless fashion (if possible). We can
> >> open ff-memless to allow such drivers to use parts of memoryless
> >> handling.
> >> 
> >> Thanks.
> > 
> > To bring this to a conclusion we could go from, would this be an
> > acceptable
> > solution?
> > 
> > - Have the HW-specific driver talk directly to ff-core and reimplement
> > upload(), play(), etc.
> > - Rewrite "ff-memless-next" so that it is not a self-contained module but
> > a
> > library of functions.
> > 
> > - Have the driver either:
> >   - Upload an effect to a device directly if the device can fully manage
> >   the
> > 
> > effect by itself.
> > 
> >   - Use provided timing functions to know when an effect should start,
> >   stop,
> > 
> > restart etc...
> > 
> >   - Use provided timing AND processing functions to combine effects that
> >   can be> 
> > combined into one, calculate periodic waveforms etc?
> > 
> > I have no problem with throwing my current approach away but before I
> > start
> > working on a new one I'd like to know which way to go...
> > 
> > Thanks,
> > Michal
> 
> Hi everyone
> 
> Allow me to introduce myself. I'm Roland Bosa and I work for Logitech,
> more specifically the gaming group. I have been tasked to look into
> gaming device support for Linux and I just started following this list
> and studying the kernel code related to the Logitech Force Feedback
> devices. The 'ff-memless-next' module sounds tailor-made for our
> devices. Please let me know how I can help with its development.


Hi,

"ff-memless-next" was designed with behavior of Logitech devices in mind, 
however it was always meant as a general replacement for the current "ff-
memless". After some followup discussion it's unlikely that it will be 
mainlined in its current form. See the discussion here 
"http://www.spinics.net/lists/linux-input/msg31426.html"; for more details.

We would however really appreciate some information regarding Logitech devices 
specifically. Although we have reverse engineered most of things, I believe 
there are still areas we're not entirely clear about. I realize that you'd 
probably have to take it up with the legal deparment but if someone at 
Logitech could at least take a look at what we've found so far at fill in the 
blanks it'd be most helpful.
Please note that we found numerous bugs and DirectInput specs violations in 
the Windows driver which might have impacted our understading of how the 
driver works. "ff-memless-next" and "lg4ff" tries to fix or work around these. A 
link to out-of-tree update to lg4ff which takes full advantage of "ff-memless-
next" is also provided in this thread.

Thanks for any help you can get us,
Michal
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