Re: [PATCH v1 3/6] dt-bindings: display: tegra: Document 8-bit CPU parallel interface

[Thierry Reding <thierry.reding@kernel.org>]

[-- Attachment #1: Type: text/plain, Size: 6010 bytes --]

On Wed, Sep 30, 2026 at 02:56:26PM +0300, Svyatoslav Ryhel wrote:
> ср, 30 вер. 2026 р. о 14:46 Thierry Reding <thierry.reding@kernel.org> пише:
> >
> > On Wed, Sep 30, 2026 at 01:56:40PM +0300, Svyatoslav Ryhel wrote:
> > > ср, 30 вер. 2026 р. о 13:50 Thierry Reding <thierry.reding@kernel.org> пише:
> > > >
> > > > On Wed, Sep 30, 2026 at 12:52:18PM +0300, Svyatoslav Ryhel wrote:
> > > > > ср, 30 вер. 2026 р. о 12:19 Mikko Perttunen <mperttunen@nvidia.com> пише:
> > > > > >
> > > > > > On Wednesday, September 30, 2026 4:05 PM Svyatoslav Ryhel wrote:
> > > > > > > Document 8-bit CPU parallel MIPI DBI Type B interface provided by
> > > > > > > Tegra20/30 SoCs display controller.
> > > > > > >
> > > > > > > Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>
> > > > > > > ---
> > > > > > >  .../display/tegra/nvidia,tegra-8bit-cpu.yaml  | 138 ++++++++++++++++++
> > > > > > >  1 file changed, 138 insertions(+)
> > > > > > >  create mode 100644 Documentation/devicetree/bindings/display/tegra/nvidia,tegra-8bit-cpu.yaml
> > > > > > >
> > > > > > > diff --git a/Documentation/devicetree/bindings/display/tegra/nvidia,tegra-8bit-cpu.yaml b/Documentation/devicetree/bindings/display/tegra/nvidia,tegra-8bit-cpu.yaml
> > > > > > > new file mode 100644
> > > > > > > index 0000000000000..f0dab608b2936
> > > > > > > --- /dev/null
> > > > > > > +++ b/Documentation/devicetree/bindings/display/tegra/nvidia,tegra-8bit-cpu.yaml
> > > > > > > @@ -0,0 +1,138 @@
> > > > > > > +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
> > > > > > > +%YAML 1.2
> > > > > > > +---
> > > > > > > +$id: http://devicetree.org/schemas/display/tegra/nvidia,tegra-8bit-cpu.yaml#
> > > > > > > +$schema: http://devicetree.org/meta-schemas/core.yaml#
> > > > > > > +
> > > > > > > +title: Nvidia Tegra DC based MIPI DBI Type B bridge
> > > > > > > +
> > > > > > > +maintainers:
> > > > > > > +  - Svyatoslav Ryhel <clamor95@gmail.com>
> > > > > > > +
> > > > > > > +description: The display controller in Tegra20/30 SoCs features an
> > > > > > > +  8-bit SPI interface that closely resembles the MIPI DBI Type B
> > > > > > > +  protocol and is referred to as '8-bit CPU'. Each display controller
> > > > > > > +  provides two such interfaces, which can be used to send MIPI DCS
> > > > > > > +  commands to initialize and control the panel while image data is
> > > > > > > +  transmitted via 16/18/24-line RGB.
> > > > > > > +
> > > > > > > +properties:
> > > > > > > +  compatible:
> > > > > > > +    const: nvidia,tegra-8bit-cpu
> > > > > > > +
> > > > > > > +  dc-gpios:
> > > > > > > +    description: Data/command selection pin.
> > > > > > > +    maxItems: 1
> > > > > > > +
> > > > > > > +  rw-gpios:
> > > > > > > +    description: Read/write pin.
> > > > > > > +    maxItems: 1
> > > > > > > +
> > > > > > > +  cs-gpios:
> > > > > > > +    description: Chip select pin.
> > > > > > > +    maxItems: 1
> > > > > > > +
> > > > > > > +  data-gpios:
> > > > > > > +    description: Specifies a set of 8 gpio pins used to transfer data.
> > > > > > > +    minItems: 8
> > > > > > > +    maxItems: 8
> > > > > >
> > > > > > Based on my admittedly brief research, according to the TRM the display
> > > > > > controller can drive all of these pins - of which there are two fixed
> > > > > > sets as you mention - directly. So we'd need to describe which interface
> > > > > > the display is connected to in DT, but not any GPIOs (which they really
> > > > > > aren't).
> > > > > >
> > > > >
> > > > > I am perfectly fine to not expose any gpios in the binding, if this is
> > > > > preferred. Only question, which method of interface checking would be
> > > > > preferred. I assume if primary then nothing, if secondary - boolean
> > > > > prop "nvidia,secondary"? Feel free to share your vision.
> > > >
> > > > The driver currently uses the GPIOs to program DBI commands, so I
> > > > suspect we do need some way of controlling those pins. Or is there a way
> > > > to have the display controller program the pins and send commands? That
> > > > would be much preferred because it would more accurately reflect the HW
> > > > design and possibly also simplify the driver because it doesn't need to
> > > > parse the GPIOs and then also not use the GPIO API to set the values.
> > > >
> > >
> > > From what I know, GPIOs must be used and freed after use. Sets of
> > > GPIOs are defined and remain fixed for primary and secondary
> > > interface.
> >
> > So you're saying that we need the GPIO handling in the RGB/DBI driver to
> > prevent anyone else from using these GPIOs and potentially messing with
> > the DBI communication?
> >
> 
> No, gpios are used for dbi communcation while their sfio versions are
> used to transfer RGB data. So there is no risk in external
> intereference.
> 
> > It feels like there should be a better mechanism for that than requiring
> > the DC driver to request all the GPIOs. Maybe these should be excluded
> > from the range of valid GPIOs?
> >
> 
> NO! These gpios can be used for random purposes on devices with non-RGB setups.
> 
> > We can make sure that device tree isn't going to use these on a given
> > platform, but there's still the risk of users grabbing them via sysfs or
> > the chardev API.
> >
> 
> Well, defining them in the schema as it is now and adding a note that
> gpios specified in the binding cannot be re-used/shared may be a
> solution. But overall it is impossible to reuse gpios dedicated for
> dbi by hw design. They cannot be shared or used for other purposes if
> dbi is used.

My concern was that somebody might try to use these pins as GPIOs, in
which case the GPIO and pin controllers are going to interoperate and
reprogram the pin functions, potentially making the DBI interface
malfunction.

Or is there some other hardware mechanism to prevent this from
happening?

Thierry

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 833 bytes --]