Re: [PATCH v7 3/7] of/platform: Add functional dependency link from DT bindings

From: Frank Rowand
Date: Mon Aug 19 2019 - 13:16:30 EST

On 8/15/19 6:50 PM, Saravana Kannan wrote:
> On Wed, Aug 7, 2019 at 7:06 PM Frank Rowand <frowand.list@xxxxxxxxx> 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.

A device can be added by a module that is loaded. In that case the device
was not present at late boot when the suppliers may turn off their resources.
(I am assuming the details since I have not reviewed the patches later in
the series that implement this part.)

Am I missing something?

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


< another snip >

> Thanks,
> Saravana