Re: [PATCH] drm/i915/display: Optimize panel power-on wait time

[Dibin Moolakadan Subrahmanian <dibin.moolakadan.subrahmanian@intel.com>]

On 02-07-2025 14:31, Jani Nikula wrote:
> On Tue, 01 Jul 2025, Lucas De Marchi <lucas.demarchi@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.subrahmanian@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.

>
>