linux-kernel - Re: [PATCH RESEND 2/4] drm: bridge: anx7688: Add anx7688 bridge driver support.

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Message-ID: <20191216163910.GC14502@pendragon.ideasonboard.com>
Date:   Mon, 16 Dec 2019 18:39:10 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Nicolas Boichat <drinkcat@...omium.org>
Cc:     Hsin-Yi Wang <hsinyi@...omium.org>,
        dri-devel@...ts.freedesktop.org, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Devicetree List <devicetree@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        Matthias Brugger <mbrugger@...e.com>,
        Russell King <rmk+kernel@....linux.org.uk>
Subject: Re: [PATCH RESEND 2/4] drm: bridge: anx7688: Add anx7688 bridge
 driver support.

Hello Nicolas and Hsin-Yi,

On Mon, Dec 16, 2019 at 06:19:24PM +0800, Nicolas Boichat wrote:
> On Mon, Dec 16, 2019 at 4:46 PM Hsin-Yi Wang wrote:
> > On Sat, Dec 14, 2019 at 6:38 AM Laurent Pinchart wrote:
> > > On Wed, Dec 11, 2019 at 02:19:09PM +0800, Hsin-Yi Wang 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.
> > >
> > > How about power, doesn't this chip have power supplies that potentially
> > > need to be controlled ?
> > >
> > Ideally we should add power supplies as well, but the power is
> > supplied by ec in mt8173 oak board. And we only have this board can
> > test this driver. If we add power supplies in driver we can't test it.
> 
> To clarify a bit more, this is because this chip is actually a
> TCPC+mux+HDMI=>DP converter
> (https://www.analogix.com/en/products/convertersbridges/anx7688). In
> Chromebook architecture, TCPC+mux is controlled by the EC (including
> power and other control pins), and the only reason we need a driver
> for the HDMI=>DP converter is to get the number of lanes on the DP
> side and filter out resolutions. Also, the converter is on a different
> I2C address and it could almost be considered as a separate device.
> 
> (of course we could write a kernel driver for the TCPC+mux but we'll
> leave that to others if there's ever a board that is built with the
> TCPC part connected to the AP)

Is the mux the one that is handled through a gpio-mux driver in this
series, or a different mux ? It would really, really help if you could
show a block diagram of the related hardware (including the EC), as this
is quite confusing. With every e-mail exchanged there's a bit more
information that change my understanding of the issue, I can't really
provide guidance without a full overview.

> > > > Signed-off-by: Nicolas Boichat <drinkcat@...omium.org>
> > > > Signed-off-by: Hsin-Yi Wang <hsinyi@...omium.org>
> > > > ---
> > > >  drivers/gpu/drm/bridge/Kconfig            |   9 +
> > > >  drivers/gpu/drm/bridge/Makefile           |   1 +
> > > >  drivers/gpu/drm/bridge/analogix-anx7688.c | 202 ++++++++++++++++++++++
> > > >  3 files changed, 212 insertions(+)
> > > >  create mode 100644 drivers/gpu/drm/bridge/analogix-anx7688.c
> > > >
> > > > diff --git a/drivers/gpu/drm/bridge/Kconfig b/drivers/gpu/drm/bridge/Kconfig
> > > > index 34362976cd6f..1f3fc6bec842 100644
> > > > --- a/drivers/gpu/drm/bridge/Kconfig
> > > > +++ b/drivers/gpu/drm/bridge/Kconfig
> > > > @@ -16,6 +16,15 @@ config DRM_PANEL_BRIDGE
> > > >  menu "Display Interface Bridges"
> > > >       depends on DRM && DRM_BRIDGE
> > > >
> > > > +config DRM_ANALOGIX_ANX7688
> > > > +     tristate "Analogix ANX7688 bridge"
> > > > +     select DRM_KMS_HELPER
> > > > +     select REGMAP_I2C
> > > > +     ---help---
> > > > +       ANX7688 is a transmitter to support DisplayPort over USB-C for
> > > > +       smartphone and tablets.
> > > > +       This driver only supports the HDMI to DP component of the chip.
> > > > +
> > > >  config DRM_ANALOGIX_ANX78XX
> > > >       tristate "Analogix ANX78XX bridge"
> > > >       select DRM_KMS_HELPER
> > > > diff --git a/drivers/gpu/drm/bridge/Makefile b/drivers/gpu/drm/bridge/Makefile
> > > > index 4934fcf5a6f8..7a1e0ec032e6 100644
> > > > --- a/drivers/gpu/drm/bridge/Makefile
> > > > +++ b/drivers/gpu/drm/bridge/Makefile
> > > > @@ -1,4 +1,5 @@
> > > >  # SPDX-License-Identifier: GPL-2.0
> > > > +obj-$(CONFIG_DRM_ANALOGIX_ANX7688) += analogix-anx7688.o
> > > >  obj-$(CONFIG_DRM_ANALOGIX_ANX78XX) += analogix-anx78xx.o
> > > >  obj-$(CONFIG_DRM_CDNS_DSI) += cdns-dsi.o
> > > >  obj-$(CONFIG_DRM_DUMB_VGA_DAC) += dumb-vga-dac.o
> > > > diff --git a/drivers/gpu/drm/bridge/analogix-anx7688.c b/drivers/gpu/drm/bridge/analogix-anx7688.c
> > > > new file mode 100644
> > > > index 000000000000..baaed48d6201
> > > > --- /dev/null
> > > > +++ b/drivers/gpu/drm/bridge/analogix-anx7688.c
> > > > @@ -0,0 +1,202 @@
> > > > +// SPDX-License-Identifier: GPL-2.0-only
> > > > +/*
> > > > + * ANX7688 HDMI->DP bridge driver
> > > > + *
> > > > + * Copyright 2016 Google LLC
> > > > + */
> > > > +
> > > > +#include <linux/i2c.h>
> > > > +#include <linux/module.h>
> > > > +#include <linux/regmap.h>
> > > > +#include <drm/drm_bridge.h>
> > > > +
> > > > +/* Register addresses */
> > > > +#define VENDOR_ID_REG 0x00
> > > > +#define DEVICE_ID_REG 0x02
> > > > +
> > > > +#define FW_VERSION_REG 0x80
> > > > +
> > > > +#define DP_BANDWIDTH_REG 0x85
> > > > +#define DP_LANE_COUNT_REG 0x86
> > >
> > > Are these registers defined by the ANX7688 hardware, or by the firmware
> > > running on the chip (and, I assume, developed by Google) ?
> > >
> > By firmware developed by ANX provided to Google.
> 
> We asked for these registers to be added to ANX FW, and this is the FW
> that is used by all elm/hana Chromebooks (I have no idea about other
> ANX customers...). We have facilities to update the ANX FW from
> coreboot/depthcharge on Chromebooks, but that does not really matter:
> the factory FW of all MP Chromebooks does provide these registers.

So the driver is specific to Chromebooks, it doesn't support all
ANX7688. Sweet :-(

-- 
Regards,

Laurent Pinchart