Re: [PATCH v5 0/4] USB: misc: Add onboard_usb_hub driver

Hi Krzysztof,

On Wed, Feb 10, 2021 at 10:04:51PM +0100, Krzysztof Kozlowski wrote:

On Wed, Feb 10, 2021 at 09:10:35AM -0800, Matthias Kaehlcke wrote:

quoted

This series adds the onboard_usb_hub_driver, the corresponding
device tree bindings and creation of onboard_usb_hub platform in
the xhci-plat driver during probe().

The main issue the driver addresses is that a USB hub needs to be
powered before it can be discovered. For discrete onboard hubs (an
example for such a hub is the Realtek RTS5411) this is often solved
by supplying the hub with an 'always-on' regulator, which is kind
of a hack.

It seems you are re-developing the power sequence drivers which perform
exactly this. Peter Chen some time ago was bringing power sequence to
USB devices, but I lost track where this ended up.

Some of his (and my) very old work (2017...) can be found here:
https://github.com/krzk/linux/tree/wip/odroid-u3-usb3503-pwrseq

pwrseq was brought up in the discussion about this driver, but wasn't
deemed suitable for this use case which might require more complex
configurations:

https://lore.kernel.org/patchwork/patch/1313000/#1512725

Instead of adding custom driver hiding some USB hub implementation,
power sequence seems a generic solution. What if you need to power cycle
other embedded USB device? Not a hub?

The driver could be extended to also cover other types of devices if desired.
Maybe it should be called usb-pwrseq then, even though it's not directly
related with the original pwrseq series.