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

[Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>]

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).

> Now, you may or may not want to describe the above in the devicetree,
> but this makes it sound like you're trying to work around an issue with
> the current Linux implementation.
> 

Adding MX domain to PHY in devicetree is definitely not a workaround. It is the
actual hardware representation. MX is the always on domain, and when CX collapse
happens during suspend state, it will ensure that all the analog components
(like PHY) are kept powered on. Otherwise, we will see link down issues.

But, I heard from Qcom that _only_ on this platform, MX is not backing the PCIe
PHY. I can correlate that with my encounter with PCIe issues after forcing CX
power collapse.

I haven't looked in detail on how this series fixes that issue though.

- Mani

> > Fix the power-domains assignment to stop potentially toggling the GDSC
> > unnecessarily.
> 
> Nothing is being toggled unnecessarily, and generally this is just
> another use count increment.
> 
> > Fixes: 813e83157001 ("arm64: dts: qcom: sc8280xp/sa8540p: add PCIe2-4 nodes")
> 
> So not sure a Fixes tag is warranted either.
> 
> > @@ -1895,7 +1895,7 @@ pcie3b_phy: phy@1c0e000 {
> >  			assigned-clocks = <&gcc GCC_PCIE3B_PHY_RCHNG_CLK>;
> >  			assigned-clock-rates = <100000000>;
> >  
> > -			power-domains = <&gcc PCIE_3B_GDSC>;
> > +			power-domains = <&rpmhpd SC8280XP_MX>;
> >  
> >  			resets = <&gcc GCC_PCIE_3B_PHY_BCR>;
> >  			reset-names = "phy";
> 
> Johan
> 

-- 
மணிவண்ணன் சதாசிவம்