Re: [PATCH 4/4] of: property: Avoid linking devices with circular dependencies

[Nicolas Saenz Julienne]

On Wed, 2020-04-15 at 11:52 -0700, Saravana Kannan wrote:
> On Wed, Apr 15, 2020 at 8:06 AM Nicolas Saenz Julienne
> <nsaenzjulienne@xxxxxxx> wrote:
> > When creating a consumer/supplier relationship between devices it's
> > essential to make sure they aren't supplying each other creating a
> > circular dependency.
> 
> Kinda correct. But fw_devlink is not just about optimizing probing.
> It's also about ensuring sync_state() callbacks work correctly when
> drivers are built as modules. And for that to work, circular
> "SYNC_STATE_ONLY" device links are allowed. I've explained it in a bit
> more detail here [1].

Understood.

[...]

> This only catches circular links made out of 2 devices. If we really
> needed such a function that worked correctly to catch bigger
> "circles", you'd need to recurse and it'll get super wasteful and
> ugly.

Yeah, I was kind of expecting this reply :).

> Thankfully, device_link_add() already checks for circular dependencies
> when we need it and it's much cheaper because the links are at a
> device level and not examined at a property level.
> 
> Is this a real problem you are hitting with the Raspberry Pi 4's? If
> so can you give an example in its DT where you are hitting this?

So the DT bit that triggered all this series is in
'arch/arm/boot/dts/bcm283x.dtsi'. Namely the interaction between
'cprman@7e101000' and 'dsi@7e209000.' Both are clock providers and both are
clock consumers of each other.

Well I had a second deeper look at the issue, here is how the circular
dependency breaks the boot process (A being soc, B being cprman and C being
dsi):

Device node A
	Device node B -> C
	Device node C -> B

The probe sequence is the following (with DL_FLAG_AUTOPROBE_CONSUMER):
1. A device is added, the rest of devices are siblings, nothing is done
2. B device is added, C device doesn't exist, B is added to
   'wait_for_suppliers' list with 'need_for_probe' flag set.
3. C device is added, B is picked up from 'wait_for_suppliers' list, device
   link created with B consuming from C.
4. C is then parsed, and tried to be linked with B as a consumer this time.
   This fails after testing for circular deps (by device_is_dependent()) during
   device_link_add(). This leaves C in the 'wait_for_suppliers' list *for ever*
   as every further attempt at add_link() on C will fail.

-> Ultimately this prevents C for ever being probed, which also prevents B from
   being probed. Which isn't good as B is the main clock provider of the system.

Note that B can live without C. I think some clock re-parenting will not be
accessible, but that's all.

> I'll have to NACK this patch for reasons mentioned above and in [1].
> However, I think I have a solution that should work for what I'm
> guessing is your real problem. But let me see the description of the
> real scenario before I claim to have a solution.

My intuition would be, upon getting a circular dep from device_is_dependent()
with DL_FLAG_AUTOPROBE_CONSUMER to switch need_for_probe to false on both
devices.

Regards,
Nicolas

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