Re: [PATCH] dd: Invoke one probe retry cycle after every initcall level

From: rishabhb
Date: Thu Aug 09 2018 - 18:30:43 EST

On 2018-08-06 01:53, Rafael J. Wysocki wrote:
On Fri, Aug 3, 2018 at 12:20 AM, Sodagudi Prasad
<psodagud@xxxxxxxxxxxxxx> wrote:
From: RAFAEL J. WYSOCKI <rafael@xxxxxxxxxx>
Date: Wed, Aug 1, 2018 at 2:21 PM
Subject: Re: [PATCH] dd: Invoke one probe retry cycle after every
initcall level
To: Rishabh Bhatnagar <rishabhb@xxxxxxxxxxxxxx>
Cc: "Rafael J. Wysocki" <rafael@xxxxxxxxxx>, Greg Kroah-Hartman
<gregkh@xxxxxxxxxxxxxxxxxxx>, Linux Kernel Mailing List
<linux-kernel@xxxxxxxxxxxxxxx>, ckadabi@xxxxxxxxxxxxxx,
tsoni@xxxxxxxxxxxxxx, Vikram Mulukutla <markivx@xxxxxxxxxxxxxx>

On Wed, Aug 1, 2018 at 11:18 PM, <rishabhb@xxxxxxxxxxxxxx> wrote:

On 2018-07-24 01:24, Rafael J. Wysocki wrote:

On Mon, Jul 23, 2018 at 10:22 PM, <rishabhb@xxxxxxxxxxxxxx> wrote:

On 2018-07-23 04:17, Rafael J. Wysocki wrote:

On Thu, Jul 19, 2018 at 11:24 PM, Rishabh Bhatnagar
<rishabhb@xxxxxxxxxxxxxx> wrote:

Drivers that are registered at an initcall level may have to
wait until late_init before the probe deferral mechanism can
retry their probe functions. It is possible that their
dependencies were resolved much earlier, in some cases even
before the next initcall level. Invoke one probe retry cycle
at every _sync initcall level, allowing these drivers to be
probed earlier.

Can you please say something about the actual use case this is
expected to address?

We have a display driver that depends 3 other devices to be
probed so that it can bring-up the display. Because of


not being met the deferral mechanism defers the probes for a later


even though the dependencies might be met earlier. With this


display can be brought up much earlier.


What runlevel brings up the display after the change?


After the change the display can come up after device_initcall level
The above mentioned 3 devices are probed at 0.503253, 0.505210 and


Only the first device is probed successfully. With the current
deferral mechanism the devices get probed again after late_initcall
at 9.19 and 9.35 secs. So display can only come up after 9.35 secs.
With this change the devices are re-probed successfully at 0.60 and
0.613 secs. Therefore display can come just after 0.613 secs.

OK, so why do you touch the initcall levels earlier than device_?

1) re-probe probing devices in the active list on every level help to
avoid circular dependency pending list.
2) There are so many devices which gets deferred in earlier init call
levels, so we wanted to reprobe them at every successive init call level.

Do you have specific examples of devices for which that helps?

This change helps in overall android bootup as well.

How exactly?

We have seen less no of re-probes at late_init and most of the driver's
dependency met earlier than late_init call level. It helped display and
couple of other drivers by executing the re probe work at every init level.

So I can believe that walking the deferred list on device_initcall and
maybe on device_initcall_sync may help, but I'm not quite convinced
that it matters for the earlier initcall levels.

Many of our drivers are dependent on the regulator and bus driver.
Both the regulator and bus driver are probed in the subsys_initcall level.
Now the probe of bus driver requires regulator to be working. If the probe of
bus driver happens before regulator, then bus driver's probe will be
deferred and all other device's probes which depend on bus driver will also be deferred.
The impact of this problem is reduced if we have this patch.