linux-kernel - Re: [PATCH v3 1/2] PCI: PCIe: ASPM: Introduce pcie_aspm

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Date:   Mon, 7 Oct 2019 17:34:28 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     linux-nvme <linux-nvme@...ts.infradead.org>,
        Keith Busch <kbusch@...nel.org>,
        Mario Limonciello <Mario.Limonciello@...l.com>,
        Kai-Heng Feng <kai.heng.feng@...onical.com>,
        Keith Busch <keith.busch@...el.com>,
        Christoph Hellwig <hch@....de>,
        Sagi Grimberg <sagi@...mberg.me>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Rajat Jain <rajatja@...gle.com>,
        Linux PCI <linux-pci@...r.kernel.org>,
        Heiner Kallweit <hkallweit1@...il.com>
Subject: Re: [PATCH v3 1/2] PCI: PCIe: ASPM: Introduce pcie_aspm_enabled()

[+cc Heiner]

On Thu, Aug 08, 2019 at 11:55:07PM +0200, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> Add a function checking whether or not PCIe ASPM has been enabled for
> a given device.
> 
> It will be used by the NVMe driver to decide how to handle the
> device during system suspend.
> 
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> ---
> 
> v2 -> v3:
>   * Make the new function return bool.
>   * Change its name back to pcie_aspm_enabled().
>   * Fix kerneldoc comment formatting.
> 
> -> v2:
>   * Move the PCI/PCIe ASPM changes to a separate patch.
>   * Add the _mask suffix to the new function name.
>   * Add EXPORT_SYMBOL_GPL() to the new function.
>   * Avoid adding an unnecessary blank line.
> 
> ---
>  drivers/pci/pcie/aspm.c |   20 ++++++++++++++++++++
>  include/linux/pci.h     |    3 +++
>  2 files changed, 23 insertions(+)
> 
> Index: linux-pm/drivers/pci/pcie/aspm.c
> ===================================================================
> --- linux-pm.orig/drivers/pci/pcie/aspm.c
> +++ linux-pm/drivers/pci/pcie/aspm.c
> @@ -1170,6 +1170,26 @@ static int pcie_aspm_get_policy(char *bu
>  module_param_call(policy, pcie_aspm_set_policy, pcie_aspm_get_policy,
>  	NULL, 0644);
>  
> +/**
> + * pcie_aspm_enabled - Check if PCIe ASPM has been enabled for a device.
> + * @pci_device: Target device.
> + */
> +bool pcie_aspm_enabled(struct pci_dev *pci_device)
> +{
> +	struct pci_dev *bridge = pci_upstream_bridge(pci_device);
> +	bool ret;
> +
> +	if (!bridge)
> +		return false;
> +
> +	mutex_lock(&aspm_lock);
> +	ret = bridge->link_state ? !!bridge->link_state->aspm_enabled : false;
> +	mutex_unlock(&aspm_lock);

Why do we need to acquire aspm_lock here?  We aren't modifying
anything, and I don't think we're preventing a race.  If this races
with another thread that changes aspm_enabled, we'll return either the
old state or the new one, and I think that's still the case even if we
don't acquire aspm_lock.

> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(pcie_aspm_enabled);
> +
>  #ifdef CONFIG_PCIEASPM_DEBUG
>  static ssize_t link_state_show(struct device *dev,
>  		struct device_attribute *attr,
> Index: linux-pm/include/linux/pci.h
> ===================================================================
> --- linux-pm.orig/include/linux/pci.h
> +++ linux-pm/include/linux/pci.h
> @@ -1567,8 +1567,11 @@ extern bool pcie_ports_native;
>  
>  #ifdef CONFIG_PCIEASPM
>  bool pcie_aspm_support_enabled(void);
> +bool pcie_aspm_enabled(struct pci_dev *pci_device);
>  #else
>  static inline bool pcie_aspm_support_enabled(void) { return false; }
> +static inline bool pcie_aspm_enabled(struct pci_dev *pci_device)
> +{ return false; }
>  #endif
>  
>  #ifdef CONFIG_PCIEAER
> 
> 
>