linux-kernel - Re: [PATCH 1/3] arm64: dts: qcom: sc8280xp: Fix PCIe PHY power-domains

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Date: Tue, 23 Jan 2024 22:36:14 +0530
From: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To: Johan Hovold <johan@...nel.org>
Cc: Konrad Dybcio <konrad.dybcio@...aro.org>,
	Bjorn Andersson <andersson@...nel.org>,
	Rob Herring <robh+dt@...nel.org>,
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Johan Hovold <johan+linaro@...nel.org>,
	Marijn Suijten <marijn.suijten@...ainline.org>,
	linux-arm-msm@...r.kernel.org, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
	Konrad Dybcio <konrad.dybcio@...ainline.org>
Subject: Re: [PATCH 1/3] arm64: dts: qcom: sc8280xp: Fix PCIe PHY
 power-domains

On Mon, Jan 22, 2024 at 06:36:51PM +0100, Johan Hovold wrote:
> On Mon, Jan 22, 2024 at 10:55:28PM +0530, Manivannan Sadhasivam wrote:
> > On Fri, Dec 29, 2023 at 10:33:34PM +0530, Manivannan Sadhasivam wrote:
> > > On Fri, Dec 29, 2023 at 12:24:55PM +0100, Johan Hovold wrote:
> > > > On Wed, Dec 27, 2023 at 11:28:26PM +0100, Konrad Dybcio wrote:
> > > > > The PCIe GDSCs are only related to the RCs. The PCIe PHYs on the other
> > > > > hand, are powered by VDD_MX and their specific VDDA_PHY/PLL regulators.
> > > > 
> > > > No, that does not seem to be entirely correct. I added the power-domains
> > > > here precisely because they were needed to enable the PHYs.
> > > > 
> > > > This is something I stumbled over when trying to figure out how to
> > > > add support for the second lane pair (i.e. four-lane mode), and I just
> > > > went back and confirmed that this is still the case.
> > > > 
> > > > If you try to enable one of these PHYs without the corresponding GDSC
> > > > being enabled, you end up with:
> > > > 
> > > > [   37.709324] ------------[ cut here ]------------
> > > > [   37.718196] gcc_pcie_3b_aux_clk status stuck at 'off'
> > > > [   37.718205] WARNING: CPU: 4 PID: 482 at drivers/clk/qcom/clk-branch.c:86 clk_branch_wait+0x144/0x15c
> > > > 	
> > > 
> > > Technically this patch is correct. PHYs are backed by MX domain only and not
> > > GDSCs. Only the controllers (PCIe, UFS, USB) are backed by GDSCs. The fact that
> > > you are seeing issue with PCIe Aux clock suggests me that this clock may not be
> > > applicable to the PHY but it needs to be enabled for working of the PHY somehow.
> > > I'll try to find the details on how exactly it is needed.
> > > 
> > > But if I get the answer like, "This clock is also sourced to PHY directly", then
> > > we may need to add dual power domain for PHY (both GDSC and MX).
> > > 
> > 
> > So I answer I got from Qcom is that this clock is only applicable to the PCIe
> > controller and not PHYs. On some platforms, there is a separate PCIE_PHY_AUX_CLK
> > coming from GCC that is used during L1SS state. I think that caused confusion
> > while adding PHY support for followup platforms and folks just used PCIE_AUX_CLK
> > since they couldn't find the actual PCIE_PHY_AUX_CLK.
> 
> Thanks for sorting that out.
> 
> > I've prepared a series to fix this mess, but I want to know how you end up
> > seeing the above "clk status stuck at off" issue. Is there an actual usecase for
> > powering up PHY without controller or you just experimented with it?
> 
> As I mentioned, I ran into this when experimenting with how to enable
> the "companion" PHY for four-lane support. There shouldn't be any use
> case for it (apart from using it to determine that the current
> description of the PHY resources is incomplete or incorrect).
> 

Ok. I tested by enabling the PHY clocks during qmp_pcie_clk_init() without
PCIE_GDSC. It worked for one instance of the PHY which doesn't have
PCIE_PHY_AUX_CLK, but for the PHY instance with this clock, I saw the same "clk
stuck" issue. Then checking the internal documentation revealed that this clock
needs PCIE_GDSC to become functional >.<

So to conclude, PCIE_AUX_CLK belongs to the controller and it needs GDSC. And
PCIE_PHY_AUX_CLK belongs to the PHY and it also needs GDSC.

I will just submit a series to remove the PCIE_AUX_CLK from PHY nodes. Then
in another series, I'll remove the GDSC for PHY instances that do not require
PCIE_PHY_AUX_CLK.

Hope this makes sense.

- Mani

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