linux-kernel - Re: [PATCH v2 2/2] drm/bridge: anx7688: Add anx7688 bridge driver support

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Date:   Sat, 15 Feb 2020 08:57:53 +0800
From:   Nicolas Boichat <drinkcat@...omium.org>
To:     Vasily Khoruzhick <anarsoul@...il.com>
Cc:     Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Collabora Kernel ML <kernel@...labora.com>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Hsin-Yi Wang <hsinyi@...omium.org>,
        Neil Armstrong <narmstrong@...libre.com>,
        David Airlie <airlied@...ux.ie>,
        Jonas Karlman <jonas@...boo.se>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Maxime Ripard <maxime@...no.tech>,
        Thomas Gleixner <tglx@...utronix.de>,
        Icenowy Zheng <icenowy@...c.io>,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        Torsten Duwe <duwe@...e.de>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Daniel Vetter <daniel@...ll.ch>
Subject: Re: [PATCH v2 2/2] drm/bridge: anx7688: Add anx7688 bridge driver support

On Sat, Feb 15, 2020 at 5:36 AM Vasily Khoruzhick <anarsoul@...il.com> wrote:
>
> On Thu, Feb 13, 2020 at 6:54 AM Enric Balletbo i Serra
> <enric.balletbo@...labora.com> wrote:
> >
> > From: Nicolas Boichat <drinkcat@...omium.org>
> >
> > ANX7688 is a HDMI to DP converter (as well as USB-C port controller),
> > that has an internal microcontroller.
> >
> > The only reason a Linux kernel driver is necessary is to reject
> > resolutions that require more bandwidth than what is available on
> > the DP side. DP bandwidth and lane count are reported by the bridge
> > via 2 registers on I2C.
>
> It is true only for your particular platform where usb-c part is
> managed by firmware. Pinephone has the same anx7688 but linux will
> need a driver that manages usb-c in addition to DP.
>
> I'd suggest making it MFD driver from the beginning, or at least make
> proper bindings so we don't have to rework it and introduce binding
> incompatibilities in future.

If that helps for the binding, ANX7688 is indeed a MFD (TCPC, HDMI to
DP converter, USB-C mux between USB 3.0 lanes and the DP output of the
embedded converter), with 2 I2C addresses:
- 0x2c is the TCPC/mux, used by the Embedded Controller [1] on Chrome
OS, and the code in this patch (my understanding is that lane count/BW
registers in the kernel driver below may only be available to FW on
Chromebooks).
- 0x28:
    - Used to update the embedded FW in the anx7688 (on Chrome OS we
do this in depthcharge [2]). This is a EEPROM-based FW (so even
without implementing this, it'll usually "just work").
    - Used to workaround some TCPC issues (see [1] again).

[1] EC driver: https://chromium.googlesource.com/chromiumos/platform/ec/+/refs/heads/master/driver/tcpm/anx7688.c
[2] depthcharge driver to update ANX7688 FW:
https://chromium.googlesource.com/chromiumos/platform/depthcharge/+/master/src/drivers/ec/anx7688/anx7688.c