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

Krzysztof Wilczyński
2020-11-15T06:08:51Z

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