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

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Message-ID: <b5ef9501-e07d-4150-9518-dd982518919e@collabora.com>
Date: Fri, 3 Jan 2025 10:16:57 +0100
From: AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>
To: Jianjun Wang <jianjun.wang@...iatek.com>,
 Bjorn Helgaas <bhelgaas@...gle.com>,
 Lorenzo Pieralisi <lpieralisi@...nel.org>,
 Krzysztof Wilczyński <kw@...ux.com>,
 Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
 Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>,
 Conor Dooley <conor+dt@...nel.org>, Matthias Brugger <matthias.bgg@...il.com>
Cc: Ryder Lee <ryder.lee@...iatek.com>, linux-pci@...r.kernel.org,
 linux-mediatek@...ts.infradead.org, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 Xavier Chang <Xavier.Chang@...iatek.com>
Subject: Re: [PATCH 3/5] PCI: mediatek-gen3: Disable ASPM L0s

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?

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;