linux-kernel - Re: [PATCH] PCI: Add sysfs attribute for PCI device power state

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Message-ID: <X7DF2ZyVnyIFjdC1@rocinante>
Date:   Sun, 15 Nov 2020 07:08:25 +0100
From:   Krzysztof Wilczyński <kw@...ux.com>
To:     Maximilian Luz <luzmaximilian@...il.com>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] PCI: Add sysfs attribute for PCI device power state

Hi Maximilian,

On 20-11-02 15:15:20, Maximilian Luz wrote:
> While most PCI power-states can be queried from user-space via lspci,
> this has some limits. Specifically, lspci fails to provide an accurate
> value when the device is in D3cold as it has to resume the device before
> it can access its power state via the configuration space, leading to it
> reporting D0 or another on-state. Thus lspci can, for example, not be
> used to diagnose power-consumption issues for devices that can enter
> D3cold or to ensure that devices properly enter D3cold at all.
> 
> To alleviate this issue, introduce a new sysfs device attribute for the
> PCI power state, showing the current power state as seen by the kernel.

Very nice!  Thank you for adding this.

[...]
> +/* PCI power state */
> +static ssize_t power_state_show(struct device *dev,
> +				struct device_attribute *attr, char *buf)
> +{
> +	struct pci_dev *pci_dev = to_pci_dev(dev);
> +	pci_power_t state = READ_ONCE(pci_dev->current_state);
> +
> +	return sprintf(buf, "%s\n", pci_power_name(state));
> +}
> +static DEVICE_ATTR_RO(power_state);
[...]

Curious, why did you decide to use the READ_ONCE() macro here?  Some
other drivers exposing data through sysfs use, but certainly not all.

Krzysztof