Re: [PATCH 1/2] dt-bindings: display: Add xylon logicvc bindings documentation

[Paul Kocialkowski]

Hi,

On Fri 13 Sep 19, 20:16, Rob Herring wrote:
> On Fri, Sep 13, 2019 at 4:58 PM Paul Kocialkowski
> <paul.kocialkowski@xxxxxxxxxxx> wrote:
> >
> > Hi Rob and thanks for the review!
> >
> > On Fri 13 Sep 19, 15:35, Rob Herring wrote:
> > > On Tue, Sep 10, 2019 at 05:34:08PM +0200, Paul Kocialkowski wrote:
> > > > The Xylon LogiCVC is a display controller implemented as programmable
> > > > logic in Xilinx FPGAs.
> > > >
> > > > Signed-off-by: Paul Kocialkowski <paul.kocialkowski@xxxxxxxxxxx>
> > > > ---
> > > >  .../bindings/display/xylon,logicvc.txt        | 188 ++++++++++++++++++
> > > >  1 file changed, 188 insertions(+)
> > > >  create mode 100644 Documentation/devicetree/bindings/display/xylon,logicvc.txt
> > >
> > > Consider converting this to DT schema format. See
> > > Documentation/devicetree/writing-schema.rst (.md in 5.3).
> >
> > Oh right, that would certainly be much more future-proof!
> >
> > > > diff --git a/Documentation/devicetree/bindings/display/xylon,logicvc.txt b/Documentation/devicetree/bindings/display/xylon,logicvc.txt
> > > > new file mode 100644
> > > > index 000000000000..eb4b1553888a
> > > > --- /dev/null
> > > > +++ b/Documentation/devicetree/bindings/display/xylon,logicvc.txt
> > > > @@ -0,0 +1,188 @@
> > > > +Xylon LogiCVC display controller
> > > > +
> > > > +The Xylon LogiCVC is a display controller that supports multiple layers.
> > > > +It is usually implemented as programmable logic and was optimized for use
> > > > +with Xilinx Zynq-7000 SoCs and Xilinx FPGAs.
> > > > +
> > > > +Because the controller is intended for use in a FPGA, most of the configuration
> > > > +of the controller takes place at logic configuration bitstream synthesis time.
> > > > +As a result, many of the device-tree bindings are meant to reflect the
> > > > +synthesis configuration. These do not allow configuring the controller
> > > > +differently than synthesis configuration.
> > > > +
> > > > +Layers are declared in the "layers" sub-node and have dedicated configuration.
> > > > +In version 3 of the controller, each layer has fixed memory offset and address
> > > > +starting from the video memory base address for its framebuffer. With version 4,
> > > > +framebuffers are configured with a direct memory address instead.
> > > > +
> > > > +Matching synthesis parameters are provided when applicable.
> > > > +
> > > > +Required properties:
> > > > +- compatible: Should be one of:
> > > > +  "xylon,logicvc-3.02.a-display"
> > > > +  "xylon,logicvc-4.01.a-display"
> > > > +- reg: Physical base address and size for the controller registers.
> > > > +- clocks: List of phandle and clock-specifier pairs, one for each entry
> > > > +  in 'clock-names'
> > > > +- clock-names: List of clock names that should at least contain:
> > > > +  - "vclk": The VCLK video clock input.
> > > > +- interrupts: The interrupt to use for VBLANK signaling.
> > > > +- xylon,display-interface: Display interface in use, should be one of:
> > > > +  - "lvds-4bits": 4-bit LVDS interface (C_DISPLAY_INTERFACE == 4).
> > > > +- xylon,display-colorspace: Display output colorspace in use, should be one of:
> > > > +  - "rgb": RGB colorspace (C_DISPLAY_COLOR_SPACE == 0).
> > > > +- xylon,display-depth: Display output depth in use (C_PIXEL_DATA_WIDTH).
> > > > +- xylon,row-stride: Fixed number of pixels in a framebuffer row (C_ROW_STRIDE).
> > > > +- xylon,layers-count: The number of available layers (C_NUM_OF_LAYERS).
> > >
> > > Presumably some of this is determined by the display attached. Isn't it
> > > safe to assume the IP was configured correctly for the intended display
> > > and you can just get this from the panel?
> >
> > Layers are what corresponds to DRM planes, which are not actually indicated
> > by the panel but are a charasteristic of the display controller. In our case,
> > this is directly selected at bitstream synthesis time for the controller.
> >
> > So I'm afraid there is no way we can auto-detect this from the driver.
> 
> Sorry, I referring to the set of properties above. In particular,
> xylon,display-interface and xylon,display-colorspace, though I don't
> know if the latter is talking in memory format or on the wire format.

Both of these are about the wire format, which is also "hardcoded" at synthesis
time with no way to be detected afterwards, as far as I know. Memory format is
described in the layer sub-nodes.

> Actually for xylon,layers-count, You should just count the child nodes
> of 'layers'.

Oh that's a good point, thanks!

> > > > +Optional properties:
> > > > +- memory-region: phandle to a node describing memory, as specified in:
> > > > +  Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt
> > > > +- clock-names: List of clock names that can optionally contain:
> > > > +  - "vclk2": The VCLK2 doubled-rate video clock input.
> > > > +  - "lvdsclk": The LVDS clock.
> > > > +  - "lvdsclkn": The LVDS clock inverted.
> > >
> > > How are these really optional?
> >
> > Well, the controller currently only supports LVDS, but more interfaces may be
> > added later, so the lvdsclk clock will be optional when another interface
> > is used instead. Maybe I'm mistaken about how to categorize them though.
> >
> > My understanding is that the need for vclk2 and lvdsclkn depend on the target
> > FPGA family. I've developped the driver without the need for them, but the
> > datasheet states that they may be needed (but doesn't provide significant
> > details about their role though).
> 
> Not sure what to tell you then. You'll see it becomes a bit messy to
> describe in schema. Ideally we define the exact number, order, and
> values possible (or sets of those).

I'll try to do my best.

> > > > +- xylon,syscon: Syscon phandle representing the logicvc instance.
> > > > +- xylon,dithering: Dithering module is enabled (C_XCOLOR).
> > > > +- xylon,background-layer: The last layer is used to display a black background
> > > > +  (C_USE_BACKGROUND). It must still be registered.
> > > > +- xylon,layers-configurable: Configuration of layers' size, position and offset
> > > > +  is enabled (C_USE_SIZE_POSITION).
> > >
> > > I would think this will effectively have to be enabled to make this
> > > usable with DRM. I'm not sure if a "standard" userspace would use any of
> > > the layers if all this is fixed.
> >
> > I was going with the same assumption, but drm_atomic_helper_check_plane_state
> > has a can_position parameter, which will check that the plane covers the
> > whole CRTC if set to false. So I guess it is somewhat expected that this can
> > be the case and some drivers (e.g. arm/hdlcd_crtc.c) also set this to false.
> 
> Certainly atomic can fail on anything not supported. My question is
> more whether userspace has some minimum requirements. A cursor
> couldn't deal with can_position=false for example.

Right, so I suppose that using an overlay plane as cursor wouldn't work
in this situation. Well, I haven't found any formal definition of what minimal
requirements are expected from overlay planes. I would expect userspace that
tries to use an overlay plane as a cursor to have a software fallback as soon
as something goes wrong. My feeling is that overlay planes are provided on a
"best-effort" basis, though contradiction is welcome here.

> > I guess this falls under a more generic discussion of what should be expected
> > by userspace when it comes to DRM. Since drivers may reject commits because of
> > any hardware-specific limitation, there is definitely a significant grey area
> > there and apparently no common rule.
> >
> > > > +
> > > > +Required sub-nodes:
> > > > +- layers: The description of the display controller layers, containing layer
> > > > +  sub-nodes that each describe a registered layer.
> > > > +- ports: The LogiCVC connection to an encoder input port. The connection
> > > > +  is modeled using the OF graph bindings, as specified in:
> > > > +  Documentation/devicetree/bindings/graph.txt
> > > > +
> > > > +Required layer properties:
> > > > +- reg: Layer index (from front to back, starting at 0).
> > > > +- xylon,layer-depth: Layer depth in use (C_LAYER_0_DATA_WIDTH).
> > > > +- xylon,layer-colorspace: Layer colorspace in use, should be one of:
> > > > + - "rgb": RGB colorspace (C_LAYER_*_TYPE == 0).
> > >
> > > Why is this needed if there's only 1?
> >
> > The hardware supports more but support is no implemented yet, so the binding
> > document may be enriched along with the driver in the future.
> >
> > Should I describe other colorspaces even if they are not currently supported?
> 
> Document what the h/w supports to the extent you can. Then we can make
> better decisions.

Okay then, I'll include those well-known other possibilities in the description.
Hopefully users will take the description for what it is and refrain from
expecting that the driver currently supports all that is described.

Thanks for the follow-up!

Cheers,

Paul

> > > > +- xylon,layer-alpha-mode: Alpha mode for the layer, should be one of:
> > > > + - "layer": Alpha is configured layer-wide (C_LAYER_*_ALPHA_MODE == 0).
> > > > + - "pixel": Alpha is configured per-pixel (C_LAYER_*_ALPHA_MODE == 1).
> > >
> > > Could just be boolean.
> >
> > In this instance too, there are other modes that are not yet implemented but
> > supported by the hardware, so I did not describe them yet but they may be added
> > later.
> >
> > > > +- xylon,layer-base-offset: offset in number of lines (C_LAYER_*_OFFSET) starting
> > > > +  from the video RAM base (C_VMEM_BASEADDR), only for version 3.
> > > > +- xylon,layer-buffer-offset: offset in number of lines (C_BUFFER_*_OFFSET)
> > > > +  starting from the layer base offset for the second buffer used in
> > > > +  double-buffering.
> > >
> > > It might be better to define all this in terms of byte offsets. I'd
> > > guess that is what CPU accesses are going to need.
> >
> > I agree that it is more convenient from a driver's perspective, but the
> > rationale is that this allows copying parameters directly from the synthesis
> > configuration file, where these are expressed as a number of lines.
> >
> > I would like to keep both delcarations as close to eachother as possible, as to
> > facilitate integration work for future users of the driver. But maybe this is a
> > bit too much in that case. What do you think?
> 
> Fair enough. I'd feel differently if I thought this would be common,
> but this seems to be a pretty specific usecase. I guess there could be
> other fixed at synthesis h/w.
> 
> Rob

-- 
Paul Kocialkowski, Bootlin
Embedded Linux and kernel engineering
https://bootlin.com

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