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:

quoted

Hi Dmitry,

thank you for reviewing this.

On Tuesday 13 of May 2014 23:38:06 Dmitry Torokhov wrote:

quoted

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.

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.

Well, these effects are not exactly managed by the device. The only thing that 
is uploaded to the device are parameters of the force to be generated. Other 
parameters - such as timing - still have to be managed by the driver. Any 
driver supporting conditional effects would then have to reimplement at least 
the timing logic.

Another thing of concern is rate limiting. During our testing we have 
discovered that some games can fire off FF commands at a very fast rate - much 
faster than USB polling rate of a device. This eventually overfills the USB 
submit queue and messes everything up. A proper way to fix this would be to 
limit the rate at which the driver sends HW requests. We already have a few 
ideas as to how to implement this in ff-memless-next. If we had the driver use 
ff-memless-next to manage one category of effects and use its own logic to 
manage the rest it'd be next to impossible to do this properly.

It should also be noted that ff-memless-next almost passes the conditional 
effects through to the HW-specific driver that then takes care of everything. A 
practical example of how this works can be found in an experimental port of 
"hid-lg4ff" driver to ff-memless-next by Edwin Velds (https://github.com/edwin-v/linux-hid-lg4ff-next). The only thing that ff-memless-next does is that it 
tells the HW-specific driver that such an effect such start or stop playing.

The reasoning above made me implement the support for conditional effects in 
the way I did. As much as I agree that we're not dealing with purely 
memoryless devices here, I believe that most more advanced FFB wheels/sticks 
are in fact "semi-memoryless" and will therefore benefit from the approach used 
in ff-memless-next.

Michal
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