Date:   Fri, 26 Aug 2022 12:01:33 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     "David E. Box" <david.e.box@...ux.intel.com>
Cc:     nirmal.patel@...ux.intel.com, jonathan.derrick@...ux.dev,
        lorenzo.pieralisi@....com, hch@...radead.org, kw@...ux.com,
        robh@...nel.org, bhelgaas@...gle.com,
        michael.a.bottini@...ux.intel.com, rafael@...nel.org,
        me@...ityamohan.in, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V6 3/3] PCI: vmd: Configure PCIe ASPM and LTR

On Mon, Feb 28, 2022 at 08:19:43PM -0800, David E. Box wrote:
> PCIe ports reserved for VMD use are not visible to BIOS and therefore not
> configured to enable PCIE ASPM.

> Additionally, PCIE LTR values may be left unset since BIOS will set
> a default maximum LTR value on endpoints to ensure that they don't
> block SoC power management.

If the ports aren't visible to BIOS, I assume BIOS doesn't configure
*anything*, including LTR.  This sentence seems like it has a little
too much information; if BIOS doesn't see the ports, LTR, SoC power
management, etc., is not relevant.

> Lack of this programming results in high power consumption on
> laptops as reported in bugzilla [1].

> For currently affected products, use pci_enable_default_link_state to set
> the allowed link states for devices on the root ports.

"Currently affected products" makes me wonder about the *other*
products?  Seems like we should handle *all* VMD devices the same way.

> Also set the LTR value to the maximum value needed for the SoC. Per
> the VMD hardware team future products using VMD will enable BIOS
> configuration of these capabilities. This solution is a workaround
> for current products that mainly targets laptops.

I guess the cover letter has a little more background on this,
although I don't understand how talking to the Intel BIOS team can
solve this for *all* vendors using these parts.

> Support is not provided if a switch used nor for hotplug.

What switch are you referring to?  What is the hotplug scenario?  Are
VMD ports hot-pluggable?  I assumed they were built into the Root
Complex and not hot-pluggable.

s/PCIE/PCIe/ several times above so they're all consistent.

s/pci_enable_default_link_state/pci_enable_default_link_state()/ so it
looks like a function.

That's a big block of text; maybe could be 2-3 paragraphs.

> [1] https://bugzilla.kernel.org/show_bug.cgi?id=213717
> 
> Signed-off-by: Michael Bottini <michael.a.bottini@...ux.intel.com>
> Signed-off-by: David E. Box <david.e.box@...ux.intel.com>
> ---
>  V6
>   - Set ASPM first before setting LTR. This is needed because some
>     devices may only have LTR set by BIOS and not ASPM
>   - Skip setting the LTR if the current LTR in non-zero.
>  V5
>   - Provide the LTR value as driver data.
>   - Use DWORD for the config space write to avoid PCI WORD access bug.
>   - Set ASPM links firsts, enabling all link states, before setting a
>     default LTR if the capability is present
>   - Add kernel message that VMD is setting the device LTR.
>  V4
>   - Refactor vmd_enable_apsm() to exit early, making the lines shorter
>     and more readable. Suggested by Christoph.
>  V3
>   - No changes
>  V2
>   - Use return status to print pci_info message if ASPM cannot be enabled.
>   - Add missing static declaration, caught by lkp@...el.com
> 
>  drivers/pci/controller/vmd.c | 66 +++++++++++++++++++++++++++++++++---
>  1 file changed, 62 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/pci/controller/vmd.c b/drivers/pci/controller/vmd.c
> index cde6e2cba210..8525bb8312f2 100644
> --- a/drivers/pci/controller/vmd.c
> +++ b/drivers/pci/controller/vmd.c
> @@ -67,10 +67,19 @@ enum vmd_features {
>  	 * interrupt handling.
>  	 */
>  	VMD_FEAT_CAN_BYPASS_MSI_REMAP		= (1 << 4),
> +
> +	/*
> +	 * Enable ASPM on the PCIE root ports and set the default LTR of the
> +	 * storage devices on platforms where these values are not configured by
> +	 * BIOS. This is needed for laptops, which require these settings for
> +	 * proper power management of the SoC.
> +	 */
> +	VMD_FEAT_BIOS_PM_QUIRK		= (1 << 5),
>  };
>  
>  struct vmd_device_data {
>  	enum vmd_features features;
> +	u16 ltr;
>  };
>  
>  static DEFINE_IDA(vmd_instance_ida);
> @@ -714,6 +723,45 @@ static void vmd_copy_host_bridge_flags(struct pci_host_bridge *root_bridge,
>  	vmd_bridge->native_dpc = root_bridge->native_dpc;
>  }
>  
> +/*
> + * Enable ASPM and LTR settings on devices that aren't configured by BIOS.
> + */
> +static int vmd_pm_enable_quirk(struct pci_dev *pdev, void *userdata)
> +{
> +	struct vmd_device_data *info = userdata;
> +	u32 ltr_reg;
> +	int pos;
> +
> +	if (!(info->features & VMD_FEAT_BIOS_PM_QUIRK))
> +		return 0;
> +
> +	pci_enable_default_link_state(pdev, PCIE_LINK_STATE_ALL);
> +
> +	pos = pci_find_ext_capability(pdev, PCI_EXT_CAP_ID_LTR);
> +	if (!pos)
> +		return 0;
> +
> +	/*
> +	 * Skip if the max snoop LTR is non-zero, indicating BIOS has set it
> +	 * so the LTR quirk is not needed.
> +	 */
> +	pci_read_config_dword(pdev, pos + PCI_LTR_MAX_SNOOP_LAT, &ltr_reg);
> +	if (!!(ltr_reg & (PCI_LTR_VALUE_MASK | PCI_LTR_SCALE_MASK)))
> +		return 0;
> +
> +	/*
> +	 * Set the default values to the maximum required by the platform to
> +	 * allow the deepest power management savings. Write as a DWORD where
> +	 * the lower word is the max snoop latency and the upper word is the
> +	 * max non-snoop latency.
> +	 */
> +	ltr_reg = (info->ltr << 16) | info->ltr;

The fact that you have to hard-code the LTR values in the driver seems
problematic because it requires updates for every new device.  I guess
you have to update the driver anyway to add Device IDs.

But surely there should be a firmware interface to discover this
platform-specific information?  Does the _DSM for Latency Tolerance
Reporting (PCI Firmware spec r3.3, sec 4.6.6) supply this? 

We badly need generic support for that _DSM, but the documentation is
somewhat lacking.

> +	pci_write_config_dword(pdev, pos + PCI_LTR_MAX_SNOOP_LAT, ltr_reg);
> +	pci_info(pdev, "VMD: Default LTR set\n");
> +
> +	return 0;
> +}
> +
>  static int vmd_enable_domain(struct vmd_dev *vmd, struct vmd_device_data *info)
>  {
>  	struct pci_sysdata *sd = &vmd->sysdata;
> @@ -867,6 +915,8 @@ static int vmd_enable_domain(struct vmd_dev *vmd, struct vmd_device_data *info)
>  		pci_reset_bus(child->self);
>  	pci_assign_unassigned_bus_resources(vmd->bus);
>  
> +	pci_walk_bus(vmd->bus, vmd_pm_enable_quirk, info);
> +
>  	/*
>  	 * VMD root buses are virtual and don't return true on pci_is_pcie()
>  	 * and will fail pcie_bus_configure_settings() early. It can instead be
> @@ -1016,28 +1066,36 @@ static const struct pci_device_id vmd_ids[] = {
>  		(kernel_ulong_t)&(struct vmd_device_data) {
>  			.features = VMD_FEAT_HAS_MEMBAR_SHADOW_VSCAP |
>  				    VMD_FEAT_HAS_BUS_RESTRICTIONS |
> -				    VMD_FEAT_OFFSET_FIRST_VECTOR,
> +				    VMD_FEAT_OFFSET_FIRST_VECTOR |
> +				    VMD_FEAT_BIOS_PM_QUIRK,
> +			.ltr = 0x1003, /* 3145728 ns */
>  		},
>  	},
>  	{ PCI_VDEVICE(INTEL, 0x4c3d),
>  		(kernel_ulong_t)&(struct vmd_device_data) {
>  			.features = VMD_FEAT_HAS_MEMBAR_SHADOW_VSCAP |
>  				    VMD_FEAT_HAS_BUS_RESTRICTIONS |
> -				    VMD_FEAT_OFFSET_FIRST_VECTOR,
> +				    VMD_FEAT_OFFSET_FIRST_VECTOR |
> +				    VMD_FEAT_BIOS_PM_QUIRK,
> +			.ltr = 0x1003, /* 3145728 ns */
>  		},
>  	},
>  	{ PCI_VDEVICE(INTEL, 0xa77f),
>  		(kernel_ulong_t)&(struct vmd_device_data) {
>  			.features = VMD_FEAT_HAS_MEMBAR_SHADOW_VSCAP |
>  				    VMD_FEAT_HAS_BUS_RESTRICTIONS |
> -				    VMD_FEAT_OFFSET_FIRST_VECTOR,
> +				    VMD_FEAT_OFFSET_FIRST_VECTOR |
> +				    VMD_FEAT_BIOS_PM_QUIRK,
> +			.ltr = 0x1003, /* 3145728 ns */
>  		},
>  	},
>  	{ PCI_VDEVICE(INTEL, PCI_DEVICE_ID_INTEL_VMD_9A0B),
>  		(kernel_ulong_t)&(struct vmd_device_data) {
>  			.features = VMD_FEAT_HAS_MEMBAR_SHADOW_VSCAP |
>  				    VMD_FEAT_HAS_BUS_RESTRICTIONS |
> -				    VMD_FEAT_OFFSET_FIRST_VECTOR,
> +				    VMD_FEAT_OFFSET_FIRST_VECTOR |
> +				    VMD_FEAT_BIOS_PM_QUIRK,
> +			.ltr = 0x1003, /* 3145728 ns */
>  		},
>  	},
>  	{ }
> -- 
> 2.25.1
> 

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