Message-ID: <20140514180558.GB30089@core.coreip.homeip.net>
Date:	Wed, 14 May 2014 11:05:58 -0700
From:	Dmitry Torokhov <dmitry.torokhov@...il.com>
To:	Michal Malý <madcatxster@...oid-pointer.net>
Cc:	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
	jkosina@...e.cz, elias.vds@...il.com, anssi.hannula@....fi,
	simon@...gewell.org
Subject: Re: [PATCH v4 01/24] input: Add ff-memless-next module

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.

-- 
Dmitry
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