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

[Nicolas Boichat <drinkcat@chromium.org>]

On Mon, Dec 16, 2019 at 4:46 PM Hsin-Yi Wang <hsinyi@chromium.org> wrote:
>
> On Sat, Dec 14, 2019 at 6:38 AM Laurent Pinchart
> <laurent.pinchart@ideasonboard.com> wrote:
> >
> > Hi Hsin-Yi and Nicolas,
> >
> > Thank you for the patch.
> >
> > On Wed, Dec 11, 2019 at 02:19:09PM +0800, Hsin-Yi Wang wrote:
> > > From: Nicolas Boichat <drinkcat@chromium.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)

> > > Signed-off-by: Nicolas Boichat <drinkcat@chromium.org>
> > > Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.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.

Thanks.