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:

quoted

+
+/** 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.

quoted

+ * hardware-specific driver returns 0, the upload is considered
successful. + * START_UNCOMB and STOP_UNCOMB commands cannot fail and the
device must always + * start the playback of the requested effect if the
UPLOAD command of the + * respective effect has been successful.
ff-memless-next will never send + * a START/STOP_UNCOMB command for an
effect that has not been uploaded + * successfully, nor will it send an
ERASE command for an effect that is + * playing (= has been started with
START_UNCOMB command).

+ */
+
+enum mlnx_commands {
+	/* Start or update a combined effect. This command is sent whenever
+	 * a FF_CONSTANT, FF_PERIODIC or FF_RAMP is started, stopped or
+	 * updated by userspace, when the applied envelopes are recalculated
+	 * or when periodic effects are recalculated. */
+	MLNX_START_COMBINED,
+	/* Stop combined effect. This command is sent when all combinable
+	 * effects are stopped. */
+	MLNX_STOP_COMBINED,
+	/* Start or update a rumble effect. This command is sent whenever
+	 * a FF_RUMBLE effect is started or when its magnitudes or directions
+	 * change. */
+	MLNX_START_RUMBLE,
+	/* Stop a rumble effect. This command is sent when all FF_RUMBLE
+	 * effects are stopped. */
+	MLNX_STOP_RUMBLE,
+	/* Start or update an uncombinable effect. This command is sent
+	 * whenever an uncombinable effect is started or updated. */

Why do we have make a distinction between rumble and all other effects?

Because "combined" effect consists of two vectors pointing along X and Y axes 
whereas "rumble" effect is a rotation speed of a rumble motor and the direction 
of rotation. Naturally these effects have to be handled in a different fashion. 
It is also possible to have a device with both rumble motors and FF actuator. 
Having such a distinction also makes it easier to handle emulation of rumble 
effects through constant force and vice versa.

Michal
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