interview question how does application connects to device

From: erkrishna@gmail.com (er krishna)
To: kernelnewbies@lists.kernelnewbies.org
Subject: interview question how does application connects to device
Date: Tue, 5 Jul 2011 13:42:53 +0530	[thread overview]
Message-ID: <CAMGjyMvRY+ipaFeysAnP1+-v81iQOvDnTzc4-7PYhkSXxidMLA@mail.gmail.com> (raw)
In-Reply-To: <CAJQzqVzVbiZMN4pQqjo+011OzpnchYUALubUjH=mRt6czNZrUw@mail.gmail.com>

On Tue, Jul 5, 2011 at 1:21 PM, Abhijit Pawar <apawar.linux@gmail.com>wrote:

>
>
> On 5 July 2011 13:01, Mandeep Sandhu <mandeepsandhu.chd@gmail.com> wrote:
>
>> Oops, accidentally pressed send...
>>
>> On Tue, Jul 5, 2011 at 12:58 PM, Mandeep Sandhu
>> <mandeepsandhu.chd@gmail.com> wrote:
>> > On Tue, Jul 5, 2011 at 12:29 PM, Paraneetharan Chandrasekaran
>> > <paraneetharanc@gmail.com> wrote:
>> >> I think the thread originator is asking about how the application knows
>> >> which device file to read or write.
>> >> This is done by h/w management system udev. udev creates/manages device
>> >> nodes in /dev/ dir and notifes applications based on the udev rules
>> written
>> >> (via HAL events or DBUS signals).
>> >
>> > I don't think udev is involved in the read/write file ops. Udev is
>> > responsible for handling hotplug events, doing certain actions based
>> > on events (as indicated by udev rules),persistent naming of devices
>> > etc...but not file i/o.
>> >
>> > That, I think, is handled by the VFS layer. Each device node is
>> > uniquely identified by it's MAJOR-MINOR number combo. I guess the VFS
>> > layer uses this to pick the correct file-ops struct to communicate
>> > with the device.
>>
>> >Eg; when we try to open a device, say /dev/ttyS0, it's major-minor
>> >numbers (eg: 64-4 on my machine) are used to lookup the file-ops
>> >struct and from then on, the VFS passes the read/write calls to this
>> >device driver.
>>
>
> Yes. whenever we read or write to the device, the file operations structure
> would be invoked for the device. And this device is identified by the Major
> and Minor number combination.
> The device driver / kernel module at registration time mention what major
> and minor it would be servicing.
>
> This is what I think it should work. Good to get confirmation though.
>

Udev creates the device node and loads the corresponding driver based on the
major/minor no. So after this step udev is out of the picture and control
goes on particular device node and then file-operation will come into
picture for read and write. It will try to read the data from cache (buffer
cache) and if not avilable there then from disk...

>
> Regards,
> Abhijit
>
>>
>> HTH,
>> -mandeep
>> >
>> > My info is a little dated, so plz CMIIW.
>> >
>> > HTH,
>> > -mandeep
>> >
>>
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>
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