From mboxrd@z Thu Jan 1 00:00:00 1970 From: Aniroop Mathur Subject: Re: [Question: Drivers/input/evdev.c] What is the use of write function in evdev_fops? Date: Fri, 15 Aug 2014 02:01:48 +0530 Message-ID: References: <20140813170639.GA16164@core.coreip.homeip.net> <20140813185851.GA34103@core.coreip.homeip.net> <8453fbc4-dfb7-4d93-acc9-e0151c8901ed@email.android.com> <20140814201903.GB13458@core.coreip.homeip.net> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Return-path: Received: from mail-la0-f44.google.com ([209.85.215.44]:55020 "EHLO mail-la0-f44.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754116AbaHNUbu (ORCPT ); Thu, 14 Aug 2014 16:31:50 -0400 Received: by mail-la0-f44.google.com with SMTP id el20so1675452lab.3 for ; Thu, 14 Aug 2014 13:31:48 -0700 (PDT) In-Reply-To: <20140814201903.GB13458@core.coreip.homeip.net> Sender: linux-input-owner@vger.kernel.org List-Id: linux-input@vger.kernel.org To: Dmitry Torokhov Cc: "linux-input@vger.kernel.org" On Fri, Aug 15, 2014 at 1:49 AM, Dmitry Torokhov wrote: > On Fri, Aug 15, 2014 at 12:55:35AM +0530, Aniroop Mathur wrote: >> On Thu, Aug 14, 2014 at 1:12 AM, Dmitry Torokhov >> wrote: >> > On August 13, 2014 12:28:46 PM PDT, Aniroop Mathur wrote: >> >>On Thu, Aug 14, 2014 at 12:50 AM, Dmitry Torokhov >> >> wrote: >> >>> On August 13, 2014 12:10:16 PM PDT, Aniroop Mathur >> >> wrote: >> >>>>On Thu, Aug 14, 2014 at 12:28 AM, Dmitry Torokhov >> >>>> wrote: >> >>>>> On Wed, Aug 13, 2014 at 11:41:20PM +0530, Aniroop Mathur wrote: >> >>>>>> Hello Mr. Torokhov :) >> >>>>>> >> >>>>>> On Wed, Aug 13, 2014 at 10:36 PM, Dmitry Torokhov >> >>>>>> wrote: >> >>>>>> > Hi Aniroop, >> >>>>>> > >> >>>>>> > On Wed, Aug 13, 2014 at 10:16:34PM +0530, Aniroop Mathur wrote: >> >>>>>> >> Dear Mr. Torokhov and Linux-Input Community, >> >>>>>> >> Greetings of the day !! :) >> >>>>>> >> >> >>>>>> >> I have not seen some good use of write function in input >> >>>>subsystem. >> >>>>>> >> I am trying find the good uses of write function in Input >> >>>>subsystem, >> >>>>>> >> but could not find the solution over internet. >> >>>>>> >> Can you please help in answering my query below: >> >>>>>> >> >> >>>>>> >> As you know, in evdev.c file, fops is defined as below >> >>>>>> >> struct file_operations evdev_fops = { >> >>>>>> >> .read = evdev_read, >> >>>>>> >> .write = evdev_write, >> >>>>>> >> ... >> >>>>>> >> } >> >>>>>> >> >> >>>>>> >> So in what cases, evdev_write function is used ? >> >>>>>> >> One case I can think of is that, it can be used in input device >> >>>>simulator >> >>>>>> >> to write the recorded data back into buffer. >> >>>>>> > >> >>>>>> > You are right, majority of times you are reading from the >> >>buffer. >> >>>>Still, >> >>>>>> > sometimes you want to control hardware state, for example, >> >>toggle >> >>>>keyboard LED. >> >>>>>> > That can be achieved by writing appropriate event to the event >> >>>>device. >> >>>>>> > >> >>>>>> >> >>>>>> Okay. :) >> >>>>>> So it means application upon receiving some key value, >> >>>>>> it can write EV_LED type of event to keyboard input device node >> >>>>>> and if dev->event function is defined in driver, driver can >> >>request >> >>>>>> hardware to toggle led. >> >>>>>> Similarly, it can be done for cases like sound (EV_SND, force >> >>>>>> feedback(EV_FF), etc >> >>>>>> Right ? >> >>>>> >> >>>>> Yes. >> >>>>> >> >>>>>> >> >>>>>> > For simulators I think uinput is suited the best. >> >>>>>> > >> >>>>>> >> >>>>>> As i know, in case of uinput, there is only one device node >> >>>>>> /dev/uinput or /dev/input/uinput. >> >>>>>> and to distinguish the events, we can use event type and code. >> >>>>>> >> >>>>>> But, if we are simulating multiple devices together like >> >>>>>> accelerometer, gyro, mag, light, compass, etc >> >>>>>> then any two devices can have same event type and code. >> >>>>>> Like accel and gyro can both have EV_REL and REL_X/Y/Z. >> >>>>>> In such a case, we won't be able to distinguish between accel and >> >>>>gyro events. >> >>>>>> >> >>>>>> Instead if we use accel and gyro separate device nodes, >> >>>>>> there is no such problem because device nodes itself are >> >>different. >> >>>>:) >> >>>>>> So for such case, I think simulation through proper device node is >> >>>>better. >> >>>>> >> >>>>> Even though there is only one /dev/input/uinput every user (an >> >>entity >> >>>>> opening that device node) will end up creating it's very own and >> >>>>> separate input device, with separate bitmasks, events, etc, etc. >> >>>>> >> >>>> >> >>>>How to use bitmasks to distinguish between two events ? >> >>>>In below code, I can only see type and code as >> >>>>identification variables. >> >>>>Can we use bitmask too here ? >> >>>> >> >>>>fd = open("/dev/uinput", O_RDWR); >> >>>> >> >>> >> >>> You need to open 2 separate file descriptors. >> >>> >> >> >> >>2 separate file descriptors like below ? >> >>int fd1 = open("/dev/uinput", O_RDWR); >> >>int fd2 = open("/dev/uinput", O_RDWR); >> >> >> >>But my reading data will still come in struct input_event as mentioned >> >>above. >> >>It has only time, type, code and value. >> >>So, how we can use bitmask here ? >> >> >> >>struct input_event { >> >>struct timeval time; >> >>__u16 type; >> >>__u16 code; >> >>__s32 value; >> >> }; >> > >> > By opening 2 fds you'll end up creating 2 separate input devices with separate evdev nodes, etc, so you will not mix up input events. >> > >> > I think at this time you should just try actually using uinput and that should clear things for you. >> > >> >> Can you please explain what do you mean by separate evdev nodes ? >> Do you mean two separate evdev nodes for uinput ? >> But as we discussed before, there is only one node in case of uinput >> i.e. /dev/uinput. > > Uinput allows to creating input devices driven from userspace. You can > create as many separate input devices as you want by opening > /dev/uinput several times since it creates a device per file descriptor. > Each of these input devices will get evdev attached to it and will get > it's own /dev/input/eventX node created. > > Really, please try using uinput, it will clear a lot if things for you. > For example, see what http://www.freedesktop.org/wiki/Evemu/ does. > Thank you Mr. Torokhov for the discussion and answering my queries. :) I will try to explore more. Good day ! > Thanks. > > -- > Dmitry