linux-kernel - Re: [PATCH 3/5] PCI: mediatek-gen3: Disable ASPM L0s

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Message-ID: <cecd7ebf0ccac9638b8e93b28fcf4df5bb81a794.camel@mediatek.com>
Date: Tue, 7 Jan 2025 02:18:55 +0000
From: Jianjun Wang (王建军) <Jianjun.Wang@...iatek.com>
To: "manivannan.sadhasivam@...aro.org" <manivannan.sadhasivam@...aro.org>,
	"conor+dt@...nel.org" <conor+dt@...nel.org>, "robh@...nel.org"
	<robh@...nel.org>, "kw@...ux.com" <kw@...ux.com>, "bhelgaas@...gle.com"
	<bhelgaas@...gle.com>, "matthias.bgg@...il.com" <matthias.bgg@...il.com>,
	"krzk+dt@...nel.org" <krzk+dt@...nel.org>, "lpieralisi@...nel.org"
	<lpieralisi@...nel.org>, AngeloGioacchino Del Regno
	<angelogioacchino.delregno@...labora.com>
CC: "linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "linux-pci@...r.kernel.org"
	<linux-pci@...r.kernel.org>, "linux-mediatek@...ts.infradead.org"
	<linux-mediatek@...ts.infradead.org>, Ryder Lee <Ryder.Lee@...iatek.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Xavier Chang (張獻文) <Xavier.Chang@...iatek.com>
Subject: Re: [PATCH 3/5] PCI: mediatek-gen3: Disable ASPM L0s

On Fri, 2025-01-03 at 10:16 +0100, AngeloGioacchino Del Regno wrote:
> External email : Please do not click links or open attachments until
> you have verified the sender or the content.
> 
> 
> Il 03/01/25 07:00, Jianjun Wang ha scritto:
> > Disable ASPM L0s support because it does not significantly save
> > power
> > but impacts performance.
> > 
> 
> That may be a good idea but, without numbers to support your
> statement, it's a bit
> difficult to say.
> 
> How much power does ASPM L0s save on MediaTek SoCs, in microwatts?
> How is the performance impacted, and on which specific device(s) on
> the PCIe bus?

It's hard to tell the exact number because it is difficult to measure,
and the number of entries into the L0s state may vary even in the same
test scenario.

However, we have encountered some compatibility issues when connected
with some PCIe EPs, and disabling the L0s can fix it. I think disabling
L0s might be the better way, since we usually use L1ss for power-saving 
when the link is idle.

Thanks.

> 
> Cheers,
> Angelo
> 
> > Signed-off-by: Jianjun Wang <jianjun.wang@...iatek.com>
> > ---
> >   drivers/pci/controller/pcie-mediatek-gen3.c | 11 +++++++++++
> >   1 file changed, 11 insertions(+)
> > 
> > diff --git a/drivers/pci/controller/pcie-mediatek-gen3.c
> > b/drivers/pci/controller/pcie-mediatek-gen3.c
> > index ed3c0614486c..4bd3b39eebe2 100644
> > --- a/drivers/pci/controller/pcie-mediatek-gen3.c
> > +++ b/drivers/pci/controller/pcie-mediatek-gen3.c
> > @@ -84,6 +84,9 @@
> >   #define PCIE_MSI_SET_ENABLE_REG             0x190
> >   #define PCIE_MSI_SET_ENABLE         GENMASK(PCIE_MSI_SET_NUM - 1,
> > 0)
> > 
> > +#define PCIE_LOW_POWER_CTRL_REG              0x194
> > +#define PCIE_FORCE_DIS_L0S           BIT(8)
> > +
> >   #define PCIE_PIPE4_PIE8_REG         0x338
> >   #define PCIE_K_FINETUNE_MAX         GENMASK(5, 0)
> >   #define PCIE_K_FINETUNE_ERR         GENMASK(7, 6)
> > @@ -458,6 +461,14 @@ static int mtk_pcie_startup_port(struct
> > mtk_gen3_pcie *pcie)
> >       val &= ~PCIE_INTX_ENABLE;
> >       writel_relaxed(val, pcie->base + PCIE_INT_ENABLE_REG);
> > 
> > +     /*
> > +      * Disable L0s support because it does not significantly save
> > power
> > +      * but impacts performance.
> > +      */
> > +     val = readl_relaxed(pcie->base + PCIE_LOW_POWER_CTRL_REG);
> > +     val |= PCIE_FORCE_DIS_L0S;
> > +     writel_relaxed(val, pcie->base + PCIE_LOW_POWER_CTRL_REG);
> > +
> >       /* Disable DVFSRC voltage request */
> >       val = readl_relaxed(pcie->base + PCIE_MISC_CTRL_REG);
> >       val |= PCIE_DISABLE_DVFSRC_VLT_REQ;
> 
>