linux-kernel - Re: [PATCH v4 06/12] PCI: qcom-ep: Gate Master AXI clock to MHI bus during L1SS

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Date:   Thu, 6 Oct 2022 03:54:58 +0530
From:   Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     lpieralisi@...nel.org, robh@...nel.org, andersson@...nel.org,
        kw@...ux.com, bhelgaas@...gle.com, linux-pci@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
        konrad.dybcio@...ainline.org, robh+dt@...nel.org,
        krzysztof.kozlowski+dt@...aro.org, devicetree@...r.kernel.org,
        dmitry.baryshkov@...aro.org,
        Krishna Chaitanya Chundru <quic_krichai@...cinc.com>
Subject: Re: [PATCH v4 06/12] PCI: qcom-ep: Gate Master AXI clock to MHI bus
 during L1SS

On Wed, Oct 05, 2022 at 05:08:38PM -0500, Bjorn Helgaas wrote:
> [+cc Krishna]
> 
> On Wed, Sep 14, 2022 at 01:23:44PM +0530, Manivannan Sadhasivam wrote:
> > During L1SS, gate the Master clock supplied to the MHI bus to save power.
> > 
> > Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
> > ---
> >  drivers/pci/controller/dwc/pcie-qcom-ep.c | 9 +++++++++
> >  1 file changed, 9 insertions(+)
> > 
> > diff --git a/drivers/pci/controller/dwc/pcie-qcom-ep.c b/drivers/pci/controller/dwc/pcie-qcom-ep.c
> > index 2dc6d4e44aff..526e98ea23f6 100644
> > --- a/drivers/pci/controller/dwc/pcie-qcom-ep.c
> > +++ b/drivers/pci/controller/dwc/pcie-qcom-ep.c
> > @@ -27,6 +27,7 @@
> >  #define PARF_SYS_CTRL				0x00
> >  #define PARF_DB_CTRL				0x10
> >  #define PARF_PM_CTRL				0x20
> > +#define PARF_MHI_CLOCK_RESET_CTRL		0x174
> >  #define PARF_MHI_BASE_ADDR_LOWER		0x178
> >  #define PARF_MHI_BASE_ADDR_UPPER		0x17c
> >  #define PARF_DEBUG_INT_EN			0x190
> > @@ -89,6 +90,9 @@
> >  #define PARF_PM_CTRL_READY_ENTR_L23		BIT(2)
> >  #define PARF_PM_CTRL_REQ_NOT_ENTR_L1		BIT(5)
> >  
> > +/* PARF_MHI_CLOCK_RESET_CTRL fields */
> > +#define PARF_MSTR_AXI_CLK_EN			BIT(1)
> > +
> >  /* PARF_AXI_MSTR_RD_HALT_NO_WRITES register fields */
> >  #define PARF_AXI_MSTR_RD_HALT_NO_WRITE_EN	BIT(0)
> >  
> > @@ -394,6 +398,11 @@ static int qcom_pcie_perst_deassert(struct dw_pcie *pci)
> >  		       pcie_ep->parf + PARF_MHI_BASE_ADDR_LOWER);
> >  	writel_relaxed(0, pcie_ep->parf + PARF_MHI_BASE_ADDR_UPPER);
> >  
> > +	/* Gate Master AXI clock to MHI bus during L1SS */
> > +	val = readl_relaxed(pcie_ep->parf + PARF_MHI_CLOCK_RESET_CTRL);
> > +	val &= ~PARF_MSTR_AXI_CLK_EN;
> > +	val = readl_relaxed(pcie_ep->parf + PARF_MHI_CLOCK_RESET_CTRL);
> 
> Is this code executed when the link actually transitions to L1.x, or
> is this just configuring things so that when the link does transition
> to L1.x sometime later, hardware will gate the Master AXI clock?
> 

It is the latter... This patch programs the EP controller in such a way that
when the link enters L1.x, the AXI clock supplied to the MHI bus (EP function)
will be cut-off to save power. Once the link goes out of L1.x, the clock will
be restored by the controller.

> Just curious because this looks more like *configuration*, i.e., the
> latter, but there's the ongoing conversation about qcom system
> suspend/resume, where IIUC, software is involved at least in some
> L1.2 exits.
> 

Krishna's suspend/resume patches are for Qcom PCIe RC controller, but this
series and this patch is for Qcom PCIe EP controller.

Thanks,
Mani

> >  	dw_pcie_ep_init_notify(&pcie_ep->pci.ep);
> >  
> >  	/* Enable LTSSM */
> > -- 
> > 2.25.1
> > 

-- 
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