On Tue, Feb 20, 2018 at 1:19 PM, Baruch Siach [off-list ref] wrote:

The Raspberry Pi 3 GPIO expander is controlled by the VC4 firmware over
I2C. The firmware mailbox interface allows the ARM core to control the
GPIO lines.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
---
v5:
  * Remove the 'firmware' property
  * Note that the gpio node is a child of the firmware node

This v5 patch applied to the GPIO tree for v4.17 with Rob's ACK.

Yours,
Linus Walleij