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

["Rafael J. Wysocki" <rafael@kernel.org>]

On Fri, Aug 3, 2018 at 12:20 AM, Sodagudi Prasad
<psodagud@codeaurora.org> wrote:
>> From: RAFAEL J. WYSOCKI <rafael@kernel.org>
>> 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@codeaurora.org>
>> Cc: "Rafael J. Wysocki" <rafael@kernel.org>, Greg Kroah-Hartman
>> <gregkh@linuxfoundation.org>, Linux Kernel Mailing List
>> <linux-kernel@vger.kernel.org>, ckadabi@codeaurora.org,
>> tsoni@codeaurora.org, Vikram Mulukutla <markivx@codeaurora.org>
>>
>>
>> On Wed, Aug 1, 2018 at 11:18 PM,  <rishabhb@codeaurora.org> wrote:
>>>
>>> On 2018-07-24 01:24, Rafael J. Wysocki wrote:
>>>>
>>>>
>>>> On Mon, Jul 23, 2018 at 10:22 PM,  <rishabhb@codeaurora.org> wrote:
>>>>>
>>>>>
>>>>> On 2018-07-23 04:17, Rafael J. Wysocki wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>> On Thu, Jul 19, 2018 at 11:24 PM, Rishabh Bhatnagar
>>>>>> <rishabhb@codeaurora.org> 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
>>
>> dependencies
>>>>>
>>>>> not being met the deferral mechanism defers the probes for a later
>>
>> time,
>>>>>
>>>>> even though the dependencies might be met earlier. With this
>>
>> change
>>>>>
>>>>> display can be brought up much earlier.
>>>>
>>>>
>>>>
>>>> OK
>>>>
>>>> What runlevel brings up the display after the change?
>>>>
>>>> Thanks,
>>>> Rafael
>>>
>>> After the change the display can come up after device_initcall level
>>> itself.
>>> The above mentioned 3 devices are probed at 0.503253, 0.505210 and
>>
>> 0.523264
>>>
>>> secs.
>>> 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.