This contains a list of properties mapping the names of unresolved symbols to lists of paths to cells within the fragments that need patching with the phandle of the target node, once that target has been located. In this case, the path is to the 0xdeadbeef value of target , but fragments can contain other unresolved references which would require additional fixes. The former is required if any node in the fragments has a phandle, because the program performing the merge will have to ensure that phandle numbers are sequential and unique. However, the latter is the key to how unresolved symbols are dealt with.
Back in section 1. This is useful because by providing sufficient aliases, we can allow an older dtc to be used to build the base DTB files. To avoid problems with old and new styles of overlay coexisting, and to match other users of overlays, the old "name-overlay. Overlays should be referred to by name alone, and the firmware or utility that loads them will append the appropriate suffix.
For example:. To avoid the need for lots of Device Tree overlays, and to reduce the need for users of peripherals to modify DTS files, the Raspberry Pi loader supports a new feature - Device Tree parameters.
This permits small changes to the DT using named parameters, similar to the way kernel modules receive parameters from modprobe and the kernel command line. It contains properties whose names are the chosen parameter names, and whose values are a sequence comprising a phandle reference to a label for the target node, and a string indicating the target property; string, integer cell and boolean properties are supported. String parameters can cause their target properties to grow, shrink, or be created. In a change from earlier implementations, integer parameters may refer to non-existent properties or to offsets beyond the end of an existing property.
Device Tree encodes boolean values as zero-length properties; if present then the property is true, otherwise it is false. They are defined like this:. Note that a property is assigned the value false by not defining it. Boolean parameters are declared like this:.
Boolean parameters can cause properties to be created or deleted.
Device Trees, overlays, and parameters
The DT parameter mechanism as described has a number of limitations, including the inability to change the name of a node and to write arbitrary values to arbitrary properties when a parameter is used. One way to overcome some of these limitations is to conditionally include or exclude certain fragments. The parameter declaration syntax has been extended to allow the otherwise illegal zero target phandle to indicate that the following string contains operations at fragment or overlay scope.
So far, four operations have been implemented:. There are some situations where it is convenient to be able to set the same value in multiple locations within the Device Tree. Rather than the ungainly approach of creating multiple parameters, it is possible to add multiple targets to a single parameter by concatenating them, like this:.
Note that it is even possible to target properties of different types with a single parameter. You could reasonably connect an "enable" parameter to a status string, cells containing zero or one, and a proper boolean property. On a Raspberry Pi it is the job of the loader one of the start. The base Device Trees are located alongside start. This selection is automatic, and allows the same SD card image to be used in a variety of devices.
As a result, passing a DT blob to a kernel that doesn't understand it causes a boot failure.
To guard against this, the loader checks kernel images for DT-compatibility, which is marked by a trailer added by the mkknlimg utility; this can be found in the scripts directory of a recent kernel source tree. Any kernel without a trailer is assumed to be non-DT-capable.
A kernel built from the rpi This configuration will cause bcmrpi-b. If these files are copied with the kernel, and if the kernel has been tagged by a recent mkknlimg , then the loader will attempt to load one of those DTBs by default. In order to manage Device Tree and overlays, the loader supports a number of new config. It will then search for parameters foo and level , and assign the indicated values to them. With a Device Tree, the kernel will automatically search for and load modules that support the indicated enabled devices.
Note, however, that layered modules such as i2c-dev still need to be loaded explicitly. The flipside is that because platform devices don't get created unless requested by the DTB, it should no longer be necessary to blacklist modules that used to be loaded as a result of platform devices defined in the board support code.
In fact, current Raspbian images ship without a blacklist file. As described above, DT parameters are a convenient way to make small changes to a device's configuration. In use, parameters look like this:. Note that multiple assignments can be placed on the same line, but ensure you don't exceed the character limit.
If you have an overlay that defines some parameters, they can be specified either on subsequent lines like this:. Note here the use of a colon : to separate the overlay name from its parameters, which is a supported syntax variant. Overlay parameters are only in scope until the next overlay is loaded. In the event of a parameter with the same name being exported by both the overlay and the base, the parameter in the overlay takes precedence; for clarity, it's recommended that you avoid doing this. To expose the parameter exported by the base DTB instead, end the current overlay scope using:.
Raspberry Pi boards have two I2C interfaces. However, there are two early revisions of the Model B that have those roles reversed. To make it possible to use one set of overlays and parameters with all Pis, the firmware creates some board-specific DT parameters. These are:. For people writing overlays, the same aliasing has been applied to the labels on the I2C DT nodes.
Thus, you should write:. As of Linux 4. Compatible kernels manage a stack of overlays that are applied on top of the base DTB. The output of a fragment shader is a depth value, a possible stencil value unmodified by the fragment shader , and zero or more color values to be potentially written to the buffers in the current framebuffers.
Fragment shaders are technically an optional shader stage. If no fragment shader is used, then the color values of the output Fragment have undefined values.
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However, the depth and stencil values for the output fragment have the same values as the inputs. This is useful for doing rendering where the only useful output is the fragment's depth, and you want to use the depth computed by the system, rather than some other depth. Such depth-only rendering is used for shadow mapping operations as well as depth pre-pass optimizations. Unlike every other shader stage, fragment shaders have implicit derivatives generated. As such, they can use the majority of the texturing functions.
You still need to watch out for non-uniform flow control. Fragment shaders also have access to the discard command. When executed, this command causes the fragment's output values to be discarded. Thus, the fragment does not proceed on to the next pipeline stages, and any fragment shader outputs are lost.
The discard command will also prevent any image store and atomic operations and Shader Storage Buffer Object writes issued before the discard from working. Normally, most of the Per-Sample Processing steps happen after the fragment shader. However, with OpenGL 4. To do this, the following syntax is used in the shader:. The inputs to the fragment shader are either generated by the system or passed from prior fixed-function operations and potentially interpolated across the surface of the primitive. The user-defined inputs received by this fragment shader will be interpolated according to the interpolation qualifiers declared on the input variables declared by this fragment shader.
The fragment shader's input variables must be declared in accord with the interface matching rules between shader stages. Specifically, between this stage and the last Vertex Processing shader stage in the program or pipeline object. Some Fragment shader built-in inputs will take values specified by OpenGL, but these values can be overridden by user control. However, the specification itself is somewhat inconsistent with this view, suggesting that the primitive ID may only get incremented based on data fed to the system, not data generated by, for example, the tessellator.
And the Vulkan specification seems to concur with this interpretation, and at least one implementation is known to agree with that as well. Until there is some clarification on the issue , you should consider the above to be questionable. The Mobile Vehicle Bay is a deployable item that allows the player to construct vehicles.
It can be crafted by using the Fabricator. It takes up nine inventory slots while in the player's inventory. When released by the player, it will float to the surface and unfold itself. It can then be boarded with LMB.