On 7/18/23 16:17, Doug Anderson wrote:
Hi,
Hi,
On Sun, Jul 9, 2023 at 6:52 AM Marek Vasut <marex@xxxxxxx> wrote:
The unprepared_time has to be initialized during probe to probe time
ktime, otherwise panel_simple_resume() panel_simple_wait() call may
wait too short time, or no time at all, which would violate the panel
timing specification. Initializing the unprepared_time() to probe time
ktime assures the delay is at least what the panel requires from the
time kernel started. The unprepared_time is then updated every time
the panel is suspended in panel_simple_suspend() too.
Fixes: e5e30dfcf3db ("drm: panel: simple: Defer unprepare delay till next prepare to shorten it")
Signed-off-by: Marek Vasut <marex@xxxxxxx>
Can you talk in more detail about the problem you're seeing? Your
patch will likely cause boot speed regressions. While correctness
trumps performance, I'd like to make sure this is right before landing
it.
With AUO T215HVN01 panel, connected to LT9211 DSI-to-LVDS bridge,
connected to MX8M Mini DSIM , the panel just would not come up correctly
because this unprepare_time is not observed. The panel would only show
blue stripe on the left side, instead of actual image.
Specifically, I think your patch is nearly the opposite as what I did
in commit 691c1fcda535 ("regulator: core: Shorten off-on-delay-us for
always-on/boot-on by time since booted"). I think many of the same
arguments I made in that commit message argue against your patch.
You cannot guarantee in which state the panel is after boot/reboot , so
I believe the kernel has to shut it down, and then bring it up, with the
correct timings.
...however, I guess in the case of the panel, things could be
different because regulators aren't directly controlled by the panel
code. Thus, I could imagine that your situation is this:
1. Bootloader runs and leaves the panel powered on.
Bootloader does not touch the panel at all.
2. Linux boots. Time starts at 0.
3. Simple fixed regulator (GPIO-based) probes and doesn't know GPIO
state of regulator, so turns it off. We'll call this time "a"
4. Panel probes at time "b" and tries to turn the panel on.
With the existing code, when we try to turn the panel code on for the
first time we'll wait "min(unprepared_time, b)". In other words, if
the panel's probe was called so early at boot that it was shorter than
unprepared_time then we'd delay. Otherwise we wouldn't. In the case
described above, this is obviously a violation.
The more correct delay would be to wait "min(unprepared_time, b-a)".
In other words, make sure the regulator is off for a certain amount of
time.
Your patch would make us always wait "unprepared_time", which is still
correct but less performant.
Did I describe your situation correctly?
Partly.
I believe the better approach would be to fix this such that we do not
operate panels out of specification right now, since panel vendors are
very sensitive about that, and any sort of optimization is a topic for
separate patch.
But please do keep in mind that depending on the state of the system in
which bootloader left it is likely a bad idea.
If so, then IMO a more
correct fix than this is actually:
a) Don't rely on the panel code to enforce your regulator constraints.
It's OK for the panel code to have this logic as a failsafe, but it's
actually better to specify "off-on-delay-us" for the regulator itself.
This means that the regulator framework can handle things correctly.
It'll work better for deferred probes and shared regulator rails,
among other things. Note that "off-on-delay-us" is currently only
implemented for fixed regulators, but IMO it would be an easy sell to
make it generic.
b) Assuming your panel is OK with it, consider using
"regulator-boot-on" to optimize your boot speed.
This is dangerous, since the panel has power sequencing requirements
which must be observed, i.e. which supplies get flipped on in specific
order with various delays between each step. That very much rules out
any such regulator-boot-on shenanigans.
...one further note is that, I believe, not all regulator drivers will
force regulators off at probe time. If your regulator is backed by a
PMIC instead of a simple fixed regulator and the bootloader left the
regulator on then I believe you could end up with a situation very
similar to the "regulator-boot-on" case.
