Putting artificial delays in a char driver
Hi All, I was fiddling around with an IR remote driver for my embedded platform to see if I can make the IR LED light (connected to GPIO's) blink on IR activity. The IR driver behaves like a char driver. The way to make the LED on/off is to simply write to some GPIO reg with a 1/0. So I was switching on the LED at the beginning of the function that reads data (in ISR context) and then switching it off at the end of that function. The problem was that all this happens too fats and I'm barely able to see the LED blink (there's only a faint flicker)! :) I tired various places in the driver to keep on/off calls as "far" as possible (ON in ISR and then OFF _after_ a copy_to_user()), but that did not help much. Whats the recommended way of solving this issue? I'm working on drivers after a really long time (last time I worked on a driver was when LDD 2nd ed was published!) so I'm not sure if bottom-halves or work-queues should be used. I was thinking what if I introduce an artificial delay in my driver where I "sleep/wait" for a few millsecs after switching on the LED and then switch it off after the incoming data has been processed. Is that a good idea? TIA, -mandeep
Hi, I can't say I have understood your scenario completely. Maybe you could turn your LED on when you want to (say in the ISR) and then start a kernel timer of sufficient duration. In the timer expiry handler, you could switch it off. By adjusting the duration, you could make sure that the LED stays on long enough to be visible. Regards, Felix.
Thanks Felix. This looks better than putting a delay in the ISR's execution path. I've implemented a simple fxn to be called by a timer after 10ms and it looks fine..as in I can see the LED blink! :) Thanks, -mandeep On Thu, May 19, 2011 at 5:21 PM, Felix Varghese <felixv1986@gmail.com> wrote:
Hi,
I can't say I have understood your scenario completely. Maybe you could turn your LED on when you want to (say in the ISR) and then start a kernel timer of sufficient duration. In the timer expiry handler, you could switch it off. By adjusting the duration, you could make sure that the LED stays on long enough to be visible.
Regards, Felix.
On Thursday 19 May 2011 13:30:33 Mandeep Sandhu wrote:
Hi All,
Whats the recommended way of solving this issue? I'm working on drivers after a really long time (last time I worked on a driver was when LDD 2nd ed was published!) so I'm not sure if bottom-halves or work-queues should be used.
Regarding bottom/top halves, modern 2.6 kernels have so called threaded interrupts. This basically means that when an IRQ is triggered, the ISR is already executing in a thread. So, if you use threaded IRQ's you don't quite have to worry about bottom/top halves of old. You can even use mutexes (not spinlocks) in the ISR. I believe that the threaded interrupts will generate some small delay before the ISR is executed, but if memory serves me right it's basically too small to matter in the most cases. May the list correct me if I'm wrong...
I was thinking what if I introduce an artificial delay in my driver where I "sleep/wait" for a few millsecs after switching on the LED and then switch it off after the incoming data has been processed. Is that a good idea?
I think that the solution described by Felix Varghese, is a quite good solution. By triggering the timer in the ISR function, it will be lit constantly if there is very much IR communications going on. BRs, /Jocke!
Regarding bottom/top halves, modern 2.6 kernels have so called threaded interrupts. This basically means that when an IRQ is triggered, the ISR is already executing in a thread. So, if you use threaded IRQ's you don't quite have to worry about bottom/top halves of old. You can even use mutexes (not spinlocks) in the ISR. I believe that the threaded interrupts will generate some small delay before the ISR is executed, but if memory serves me right it's basically too small to matter in the most cases.
Thanks for pointing this out. Didn't we now have threader IRQs! Nice.
I think that the solution described by Felix Varghese, is a quite good solution. By triggering the timer in the ISR function, it will be lit constantly if there is very much IR communications going on.
True. Using timers now. It's quite simple to use and looks clean...plus my fxn that toggles the LED states are quite simple/basic and have no resource locking requirements so no worries of race cond's too. Thanks, -mandeep
BRs, /Jocke!
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participants (3)
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Felix Varghese -
Joachim Holst -
Mandeep Sandhu