Re: [PATCH v3 2/7] regulator: qca6390: add support for QCA639x powerup sequence

From: Dmitry Baryshkov
Date: Thu Aug 12 2021 - 07:51:45 EST

On 12/08/2021 12:48, Ulf Hansson wrote:
On Tue, 10 Aug 2021 at 18:03, Bjorn Andersson
<bjorn.andersson@xxxxxxxxxx> wrote:

On Tue 10 Aug 06:55 CDT 2021, Ulf Hansson wrote:

On Wed, 14 Jul 2021 at 18:47, Rob Herring <robh@xxxxxxxxxx> wrote:

On Thu, Jul 08, 2021 at 02:37:44PM +0300, Dmitry Baryshkov wrote:

On Thu, 8 Jul 2021 at 13:10, Ulf Hansson <ulf.hansson@xxxxxxxxxx> wrote:

- Peter (the email was bouncing)

+ Peter's address

On Tue, 6 Jul 2021 at 13:55, Mark Brown <broonie@xxxxxxxxxx> wrote:

On Tue, Jul 06, 2021 at 09:54:03AM +0200, Ulf Hansson wrote:
On Tue, 22 Jun 2021 at 00:32, Dmitry Baryshkov

Qualcomm QCA6390/1 is a family of WiFi + Bluetooth SoCs, with BT part
being controlled through the UART and WiFi being present on PCIe
bus. Both blocks share common power sources. Add device driver handling
power sequencing of QCA6390/1.

Power sequencing of discoverable buses have been discussed several
times before at LKML. The last attempt [1] I am aware of, was in 2017
from Peter Chen. I don't think there is a common solution, yet.

This feels a bit different to the power sequencing problem - it's not
exposing the individual inputs to the device but rather is a block that
manages everything but needs a bit of a kick to get things going (I'd
guess that with ACPI it'd be triggered via AML). It's in the same space
but it's not quite the same issue I think, something that can handle
control of the individual resources might still struggle with this.

Well, to me it looks very similar to those resouses we could manage
with the mmc pwrseq, for SDIO. It's also typically the same kind of
combo-chips that moved from supporting SDIO to PCIe, for improved
performance I guess. More importantly, the same constraint to
pre-power on the device is needed to allow it to be discovered/probed.

In our case we'd definitely use pwrseq for PCIe bus and we can also
benefit from using pwrseq for serdev and for platform busses also (for
the same story of WiFi+BT chips).

I can take a look at rewriting pwrseq code to also handle the PCIe
bus. Rewriting it to be a generic lib seems like an easy task,
plugging it into PCIe code would be more fun.

Platform and serdev... Definitely even more fun.

I don't want to see pwrseq (the binding) expanded to other buses. If
that was the answer, we wouldn't be having this discussion. It was a
mistake for MMC IMO.

Let's make sure we get your point correctly. I think we have discussed
this in the past, but let's refresh our memories.

If I recall correctly, you are against the mmc pwrseq DT bindings
because we are using a separate pwrseq OF node, that we point to via a
"mmc-pwrseq" property that contains a phandle from the mmc controller
device node. Is that correct?

If we would have encoded the power sequence specific properties, from
within a child node for the mmc controller node, that would have been
okay for you, right?

In Dmitry's case, we have an external chip with that needs to be powered
on per a specific sequence, at which point the WiFi driver on PCIe and
BT driver on serdev will be able to communicate with the device.

Thanks for sharing more details.

So, not only do we have a discoverable device that needs to be powered
on in a device specific way before probing, but in fact we have two
consumers of that "combo chip", one (PCIe) for Wifi and one (serdev)
for Bluetooth.

The extended case of this is where we have an SDX55 modem soldered onto
the pcb next to the SoC, in which case the power sequencing is even more
complex and additionally there are incoming gpios used to detect things
such as the firmware of the modem has crashed and Linux needs to toggle
power and rescan the PCIe bus.

That sounds very similar to what we manage for the SDIO bus already.

We have a mmc pwrseq node to describe what resources that are needed
to power on/off the external chip. The driver for the functional
device (Wifi chip for example) may then call SDIO APIs provided by the
mmc core to power on/off the device, in case some kind of reset would
be needed.

Additionally, we have a child node below the mmc controller node,
allowing us to describe device specific things for the SDIO functional
device, like an out-of-band IRQ line for example.

Overall, this seems to work fine, even if the DT bindings may be questionable.

In both of these cases it seems quite reasonable to represent that
external chip (and it's power needs) as a separate DT node. But we need
a way to link the functional devices to that thing.

Don't get me wrong, I am not suggesting we should re-use the
mmc-pwrseq DT bindings - but just trying to share our experience
around them.

In the cases you describe, it certainly sounds like we need some kind
of minimal description in DT for these functional external devices.
For GPIO pins, for example.

How to describe this in DT is one thing, let's see if Rob can help to
point us in some direction of what could make sense.

When it comes to implementing a library/interface to manage these
functional devices, I guess we just have to continue to explore
various options. Perhaps just start simple with another subsystem,
like PCIe and see where this brings us.

Thank you for your opinion and suggestions. In fact I'm probably going to start working on non-discoverable busses first (by chaning support for few other BT+WiFi Qualcomm chips), later shifting the attention to the PCIe part. While this may seem like a longer path, I'd like to narrow pwrseq subsystem first, before going into PCIe details.

With best wishes