On 02-07-2025 14:31, Jani Nikula wrote:
On Tue, 01 Jul 2025, Lucas De Marchi <lucas.demar...@intel.com> wrote:
On Tue, Jul 01, 2025 at 12:28:41PM +0300, Jani Nikula wrote:
On Mon, 30 Jun 2025, Dibin Moolakadan Subrahmanian
<dibin.moolakadan.subrahman...@intel.com> wrote:
The current wait_panel_on() uses intel_de_wait() with a long timeout
(5000ms), which is suboptimal on Xe platforms where the underlying
xe_mmio_wait32() employs an exponential backoff strategy. This leads
to unnecessary delays during resume or power-on when the panel becomes
ready earlier than the full timeout.
This patch splits the total wait time into two pases
- First wait for the typical panel-on time(180ms)
- If panel is not ready , continue polling in short 20ms intervals
until the maximum timeout (5000ms) is reached
I'm *very* reluctant to merge any new custom wait hacks. I'm in the
process of *removing* a plethora of them [1][2][3].
good riddance
Yay!
I think the question is, should xe_mmio_wait32() (and the i915
counterpart) and the intel_de_wait*() functions be migrated to an
interface similar to read_poll_timeout(), i.e. provide sleep and timeout
separately, no exponential backoff. And implement the waits using
read_poll_timeout(), or whatever Ville ends up with [4].
I saw your patch series and I'm eagerly waiting it to either propagate
it in xe or have someone merge such a patch. I'm not sure about
removing the exponential backoff is a good thing overall, but if it's
needed then it needs to be justified to add a new function to pair with
read_poll_timeout(), not a custom driver function.
While I'm negative about the patch at hand, the underlying problem is
very real.
I think at the very least the exponential sleep backoff needs an upper
bound that's relative to the timeout. Maybe 10-25% of timeout?
With the example case of 5 second timeout, the exponential backoff
starting from 10 us leads to a dozen polls before reaching 100 ms
elapsed time, but then polls at approximately 1 s, 2 s, 4 s, and 8 s
elapsed time. Longer timeouts are of course rare, but they exhibit
increasingly worse behaviour.
So if what we're waiting takes 2.1 seconds, the next check will be about
2 seconds later. Similarly, if it takes 4.1 seconds, the next check will
be about 4 seconds later, in this case exceeding the timeout by 3
seconds.
Anyway, if xe_mmio_wait32() remains as it is, with read_poll_timeout()
it's trivial to do the wait in the intel_de_*() macros, in display side,
with sleeps and timeouts defined in display. Because for most things the
really quick fast polls are useless in display.
This exponential sleep back-off is causing around 120ms additional
delay in resume compared to i915.
how about polling as below , use intel_de_read and read_poll_timeout
ret = read_poll_timeout(intel_de_read, reg_val,
((reg_val & mask) == value),
(20 * 1000), // poll every 20ms
(PANEL_MAXIMUM_ON_TIME_MS * 1000), // total
timeout (us)
true,
display, pp_stat_reg);
Regards,
Dibin
BR,
Jani.
