Re: DT property keep-power-in-suspend and how WiFi drivers use it

On Wed, 29 Jan 2025 at 12:05, Matthias Proske <[email protected]> wrote:
>
> Hello,
>
> I have a question regarding the usage of the Device Tree property
> `keep-power-suspend`.
>
> In the Device Tree documentation it reads:
> "SDIO only. Preserves card power during a suspend/resume cycle."
>
> Does that mean that the SDIO Host Controller will remain powered or
> should this equally apply to anything that is connected to this SDIO
> Host Controller?

Unfortunately the documentation isn't really clear.
"keep-power-in-suspend" means that the platform is *capable* of
keeping the SDIO card powered when the system is suspended.

Depending on what the SDIO func-driver (like the brcm_fmac driver)
decides to do during system suspend/resume, the mmc core we may or may
not keep the SDIO card powered.

>
> To give a bit more background:
>
> I have an embedded board with a brcm_fmac WiFi module. It seems that due
> to a hardware limitation we are not permitted to switch the module off.
> It simply cannot be re-probed afterwards.

How did we manage to power it on and probe it in the first place?

>
> The property `keep-power-in-suspend` was used and that used to work fine
> until 92caded ("brcmfmac: Avoid keeping power to SDIO card
> unless WOWL is used"), which made the wifi adapter by default turn off
> on suspend to be re-probed on resume. Not working on our board...

Would you mind elaborating why it isn't working?

>
>
> I have also looked at other WiFi drivers with an SDIO interface and it
> seems that none of them are really honoring the `keep-power-in-suspend`
> flag. Is this flag for the SDIO Host Controller only?

See above.

>
>
> What would be proper way to implement it so that the brcm_fmac return to
> its old behaviour if necessary?
> Add a Device Tree property directly for the brcm_fmac driver..?

The corresponding SDIO func-driver may call
sdio_set_host_pm_flags(func, MMC_PM_KEEP_POWER). In this way, the mmc
core will leave the SDIO card powered-on during system suspend.
Although, unless it's really necessary, it's ofcourse a bad idea as it
would mean wasting energy when the system is suspended.

>
> I would like to write a patch, but I would like to know in which
> direction to go.
>
>
> Thank you for any feedback
>
> Matthias

Kind regards
Uffe