Tasklet usage in the DRM

["Michel Dänzer" <michel@tungstengraphics.com>]

I just read an article on LWN's kernel page about plans to remove
tasklets, and I thought I'd explain what the DRM is using tasklets for.
Maybe there's other ways to satisfy the requirements equally well or
even better.


The i915 driver uses a tasklet to make sure a GL buffer swap blit or
flip takes effect (or at least starts in the case of a blit) during the
vertical blank period, to avoid tearing. I chose a tasklet for this
purpose because:

      * The traditional method of the vertical blank interrupt waking up
        the user process, which would then emit the buffer swap
        commands, didn't even come close to avoiding tearing. So I
        suspect a workqueue wouldn't cut it either.
      * The processing of scheduled buffer swaps could potentially take
        a long time, so doing it in hardirq context could cause high IRQ
        latency.
      * It requires holding the DRM lock, so the 'each tasklet can only
        run once at a time' restriction is fine.


I'm looking forward to any suggestions what to do with this in case
tasklets disappear, and I'll gladly provide further clarification on the
requirements.


-- 
Earthling Michel Dänzer           |          http://tungstengraphics.com
Libre software enthusiast         |          Debian, X and DRI developer