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From: Frank Rowand <frowand.list@gmail.com>
To: Saravana Kannan <saravanak@google.com>
Cc: Rob Herring <robh+dt@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	Jonathan Corbet <corbet@lwn.net>,
	"open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
	<devicetree@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>,
	David Collins <collinsd@codeaurora.org>,
	Android Kernel Team <kernel-team@android.com>,
	Linux Doc Mailing List <linux-doc@vger.kernel.org>
Subject: Re: [PATCH v7 3/7] of/platform: Add functional dependency link from DT bindings
Date: Mon, 19 Aug 2019 14:30:04 -0700	[thread overview]
Message-ID: <74931824-f8a1-0435-e00a-5b5cdbe8a8a2@gmail.com> (raw)
In-Reply-To: <CAGETcx-XcXZq7YFHsFdzBDniQku9cxFUJL_vBoEKKhCH+cDKRw@mail.gmail.com>

On 8/19/19 1:49 PM, Saravana Kannan wrote:
> On Mon, Aug 19, 2019 at 10:16 AM Frank Rowand <frowand.list@gmail.com> wrote:
>>
>> On 8/15/19 6:50 PM, Saravana Kannan wrote:
>>> On Wed, Aug 7, 2019 at 7:06 PM Frank Rowand <frowand.list@gmail.com> wrote:
>>>>
>>>> On 7/23/19 5:10 PM, Saravana Kannan wrote:
>>>>> Add device-links after the devices are created (but before they are
>>>>> probed) by looking at common DT bindings like clocks and
>>>>> interconnects.
>>
>>
>> < very big snip (lots of comments that deserve answers) >
>>
>>
>>>>
>>>> /**
>>>>  * of_link_property - TODO:
>>>>  * dev:
>>>>  * con_np:
>>>>  * prop:
>>>>  *
>>>>  * TODO...
>>>>  *
>>>>  * Any failed attempt to create a link will NOT result in an immediate return.
>>>>  * of_link_property() must create all possible links even when one of more
>>>>  * attempts to create a link fail.
>>>>
>>>> Why?  isn't one failure enough to prevent probing this device?
>>>> Continuing to scan just results in extra work... which will be
>>>> repeated every time device_link_check_waiting_consumers() is called
>>>
>>> Context:
>>> As I said in the cover letter, avoiding unnecessary probes is just one
>>> of the reasons for this patch. The other (arguably more important)
>>
>> Agree that it is more important.
>>
>>
>>> reason for this patch is to make sure suppliers know that they have
>>> consumers that are yet to be probed. That way, suppliers can leave
>>> their resource on AND in the right state if they were left on by the
>>> bootloader. For example, if a clock was left on and at 200 MHz, the
>>> clock provider needs to keep that clock ON and at 200 MHz till all the
>>> consumers are probed.
>>>
>>> Answer: Let's say a consumer device Z has suppliers A, B and C. If the
>>> linking fails at A and you return immediately, then B and C could
>>> probe and then figure that they have no more consumers (they don't see
>>> a link to Z) and turn off their resources. And Z could fail
>>> catastrophically.
>>
>> Then I think that this approach is fatally flawed in the current implementation.
> 
> I'm waiting to hear how it is fatally flawed. But maybe this is just a
> misunderstanding of the problem?

Fatally flawed because it does not handle modules that add a consumer
device when the module is loaded.


> 
> In the text below, I'm not sure if you mixing up two different things
> or just that your wording it a bit ambiguous. So pardon my nitpick to
> err on the side of clarity.

Please do nitpick.  Clarity is good.


> 
>> A device can be added by a module that is loaded.
> 
> No, in the example I gave, of_platform_default_populate_init() would
> add all 3 of those devices during arch_initcall_sync().

The example you gave does not cover all use cases.

There are modules that add devices when the module is loaded.  You can not
ignore systems using such modules.


> 
>>  In that case the device
>> was not present at late boot when the suppliers may turn off their resources.
> 
> In that case, the _drivers_ for those devices aren't present at late
> boot. So that they can't request to keep the resources on for their
> consumer devices. Since there are no consumer requests on resources,
> the suppliers turn off their resources at late boot (since there isn't
> a better location as of today). The sync_state() call back added in a
> subsequent patche in this series will provide the better location.

And the sync_state() call back will not deal with modules that add consumer
devices when the module is loaded, correct?


> 
>> (I am assuming the details since I have not reviewed the patches later in
>> the series that implement this part.)
>>
>> Am I missing something?
> 
> I think you are mixing up devices getting added/populated with drivers
> getting loaded as modules?

Only some modules add devices when they are loaded.  But these modules do
exist.

-Frank

> 
>> If I am wrong, then I'll have more comments for your review replies for
>> patches 2 and 3.
> 
> I'll wait for more review replies?
> 
> Thanks,
> Saravana
> 


  reply	other threads:[~2019-08-19 21:30 UTC|newest]

Thread overview: 36+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-07-24  0:10 [PATCH v7 0/7] Solve postboot supplier cleanup and optimize probe ordering Saravana Kannan
2019-07-24  0:10 ` [PATCH v7 1/7] driver core: Add support for linking devices during device addition Saravana Kannan
2019-08-08  2:04   ` Frank Rowand
2019-08-16  1:50     ` Saravana Kannan
2019-08-19  3:38       ` Frank Rowand
2019-08-20  0:00         ` Saravana Kannan
2019-08-20  4:25           ` Frank Rowand
2019-08-20 22:10             ` Saravana Kannan
2019-08-21  1:06               ` Frank Rowand
2019-08-21  1:56                 ` Greg Kroah-Hartman
2019-08-21  2:04                   ` Saravana Kannan
     [not found]                   ` <CAGETcx8eVfJ0mn08E6pTDytjVE+RFcj_gOt_enide4E6G1NAWw@mail.gmail.com>
2019-08-21  4:24                     ` Frank Rowand
2019-08-21  2:22                 ` Saravana Kannan
2019-08-21 15:36               ` Frank Rowand
2019-07-24  0:10 ` [PATCH v7 2/7] driver core: Add edit_links() callback for drivers Saravana Kannan
2019-08-08  2:05   ` Frank Rowand
2019-08-16  1:50     ` Saravana Kannan
2019-07-24  0:10 ` [PATCH v7 3/7] of/platform: Add functional dependency link from DT bindings Saravana Kannan
2019-08-08  2:06   ` Frank Rowand
2019-08-16  1:50     ` Saravana Kannan
2019-08-19 17:16       ` Frank Rowand
2019-08-19 20:49         ` Saravana Kannan
2019-08-19 21:30           ` Frank Rowand [this message]
2019-08-20  0:09             ` Saravana Kannan
2019-08-20  4:26               ` Frank Rowand
2019-08-20 22:09                 ` Saravana Kannan
2019-08-21 16:39                   ` Frank Rowand
2019-07-24  0:10 ` [PATCH v7 4/7] driver core: Add sync_state driver/bus callback Saravana Kannan
2019-07-24  0:10 ` [PATCH v7 5/7] of/platform: Pause/resume sync state during init and of_platform_populate() Saravana Kannan
2019-07-24  0:10 ` [PATCH v7 6/7] of/platform: Create device links for all child-supplier depencencies Saravana Kannan
2019-07-24  0:11 ` [PATCH v7 7/7] of/platform: Don't create device links for default busses Saravana Kannan
2019-07-25 13:42 ` [PATCH v7 0/7] Solve postboot supplier cleanup and optimize probe ordering Greg Kroah-Hartman
2019-07-25 21:04   ` Frank Rowand
2019-07-26 14:32     ` Greg Kroah-Hartman
2019-07-31  2:22       ` Frank Rowand
2019-08-08  2:02 ` Frank Rowand

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