Hi,

On 17-10-16 23:33, Jarosław Nieć wrote:
Hi Maxime,

On Mon, Oct 17, 2016 at 9:51 PM, Maxime Ripard <maxime.rip...@free-electrons.com 
<mailto:maxime.rip...@free-electrons.com>> wrote:

    Hi Jarosław,

    On Mon, Oct 17, 2016 at 08:15:53PM +0200, Jarosław Nieć wrote:
    > I've decided that I want to learn kernel hacking a little and because of
    > that and because of new HDMI CEC Framework in kernel 4.8 I chose to write
    > CEC driver for old Allwinner A10.
    > There in no CEC hardware core for this CPU and I think the only way is to
    > create implementation that uses bit banging.
    > I've investigated the topic a little, tried few things and finally I have
    > prototype that more or less can receive and send CEC messages using this
    > new Framework.
    > Code isn't very pretty right now and there is few things missing (like
    > proper error handling etc.) but before I continue I want firstly ask you
    > guys few questions.

    I actually started to look into this too a few days ago. Nothing
    really major though :)


OK so maybe I will clean my implementation (I hope in few days) and pass it to 
first review.
You could see it and decide if it make sense to invest more time in it or start 
from scratch :)


    > 1) For bit banging I'm using high resolution timer. If CEC line is idle
    > this timer ticks every 2.4 ms (416Hz). For every tick it checks the state
    > of CEC line and when it detects start of CEC message it reduce delay to 
0.1
    > ms (10000Hz).
    > We need more or less this kind of frequency to probe CEC line to detect if
    > transmitted bit is 0 or 1. I've used high resolution timer because it was
    > the solution that give me this very accurate 0.1 ms delay.
    > I've also tried udelay, usleep_range and few other functions, but without
    > success. They were too unreliable (because triggered by scheduler) or too
    > CPU consuming (because of using busy-wait loop for too long).
    > OK so the first question is does it make sens to use HR timer for bit
    > banging? What other things I could try? Maybe you know some traps that are
    > connected with HR timers?

    I had this exact same discussion today on #v4l, and what design we
    should implement, and we came to pretty much that conclusion.


I had also another idea how this CEC could be implemented, but I don't think it 
is possible.
CEC line is connected to P23 CPU ball (for A10) and it would be very good if we 
could use this pin as a GPIO.
Implementing CEC using GPIO should be much easier than using this CEC register.
But as I mentioned I don't think it is possible, not for this pin.

BTW Implementing general cec-over-gpio driver could be also quite useful for 
lot of projects.

If all the hardware allows you to-do is set / read the pin, you could write a 
gpio-chip
driver for it, and then attach a generic cec-over-gpio driver to that, that 
seems like
a better idea then an Allwinner specific bit-bang driver.


Regards,

Hans

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