.. Copyright 2021 Simon Ser

.. contents::

linux-dmabuf feedback introduction

linux-dmabuf feedback allows compositors and clients to negotiate optimal buffer allocation parameters. This document will assume that the compositor is using a rendering API such as OpenGL or Vulkan and KMS as the presentation API: even if linux-dmabuf feedback isn't restricted to this use-case, it's the most common.

linux-dmabuf feedback introduces the following concepts:

1. A main device. This is the render device that the compositor is using to perform composition. Compositors should always be able to display a buffer submitted by a client, so this device can be used as a fallback in case none of the more optimized code-paths work. Clients should allocate buffers such that they can be imported and textured from the main device.

2. One or more tranches. Each tranche consists of a target device, allocation flags and a set of format/modifier pairs. A tranche can be seen as a set of formats/modifier pairs that are compatible with the target device.

   A tranche can have the scanout flag. It means that the target device is a KMS device, and that buffers allocated with one of the format/modifier pairs in the tranche are eligible for direct scanout.

   Clients should use the tranches in order to allocate buffers with the most appropriate format/modifier and also to avoid allocating in private device memory when cross-device operations are going to happen.

linux-dmabuf feedback implementation notes

This section contains recommendations for client and compositor implementations.

For clients

Clients are expected to either pick a fixed DRM format beforehand, or perform the following steps repeatedly until they find a suitable format.

Basic clients may only support static buffer allocation on startup. These clients should do the following:

1. Send a get_default_feedback request to get global feedback.

2. Select the device indicated by main_device for allocation.

3. For each tranche:

   1. If tranche_target_device doesn't match the allocation device, ignore the tranche.
   2. Accumulate allocation flags from tranche_flags.
   3. Accumulate format/modifier pairs received via tranche_formats in a list.
   4. When the tranche_done event is received, try to allocate the buffer with the accumulated list of modifiers and allocation flags. If that fails, proceed with the next tranche. If that succeeds, stop the loop.

4. Destroy the feedback object.

Tranches are ordered by preference: the more optimized tranches come first. As such, clients should use the first tranche that happens to work.

Some clients may have already selected the device they want to use beforehand. These clients can ignore the main_device event, and ignore tranches whose tranche_target_device doesn't match the selected device. Such clients need to be prepared for the wp_linux_buffer_params.create request to potentially fail.

If the client allocates a buffer without specifying explicit modifiers on a device different from the one indicated by main_device, then the client must force a linear layout.

Some clients might support re-negotiating the buffer format/modifier on the fly. These clients should send a get_surface_feedback request and keep the feedback object alive after the initial allocation. Each time a new set of feedback parameters is received (ended by the done event), they should perform the same steps as basic clients described above. They should detect when the optimal allocation parameters didn't change (same format/modifier/flags) to avoid needlessly re-allocating their buffers.

Some clients might additionally support switching the device used for allocations on the fly. Such clients should send a get_surface_feedback request. For each tranche, select the device indicated by tranche_target_device for allocation. Accumulate allocation flags (received via tranche_flags) and format/modifier pairs (received via tranche_formats) as usual. When the tranche_done event is received, try to allocate the buffer with the accumulated list of modifiers and the allocation flags. Try to import the resulting buffer by sending a wp_linux_buffer_params.create request (this might fail). Repeat with each tranche until an allocation and import succeeds. Each time a new set of feedback parameters is received, they should perform these steps again. They should detect when the optimal allocation parameters didn't change (same device/format/modifier/flags) to avoid needlessly re-allocating their buffers.

For compositors

Basic compositors may only support texturing the DMA-BUFs via a rendering API such as OpenGL or Vulkan. Such compositors can send a single tranche as a reply to both get_default_feedback and get_surface_feedback. Set the main_device to the rendering device. Send the tranche with tranche_target_device set to the rendering device and all of the DRM format/modifier pairs supported by the rendering API. Do not set the scanout flag in the tranche_flags event.

Some compositors may support direct scan-out for full-screen surfaces. These compositors can re-send the feedback parameters when a surface becomes full-screen or leaves full-screen mode if the client has used the get_surface_feedback request. The non-full-screen feedback parameters are the same as basic compositors described above. The full-screen feedback parameters have two tranches: one with the format/modifier pairs supported by the KMS plane, with the scanout flag set in the tranche_flags event and with tranche_target_device set to the KMS scan-out device; the other with the rest of the format/modifier pairs (supported for texturing, but not for scan-out), without the scanout flag set in the tranche_flags event, and with the tranche_target_device set to the rendering device.

Some compositors may support direct scan-out for all surfaces. These compositors can send two tranches for surfaces that become candidates for direct scan-out, similarly to compositors supporting direct scan-out for fullscreen surfaces. When a surface stops being a candidate for direct scan-out, compositors should re-send the feedback parameters optimized for texturing only. The way candidates for direct scan-out are selected is compositor policy, a possible implementation is to select as many surfaces as there are available hardware planes, starting from surfaces closer to the eye.

Some compositors may support multiple devices at the same time. If the compositor supports rendering with a fixed device and direct scan-out on a secondary device, it may send a separate tranche for surfaces displayed on the secondary device that are candidates for direct scan-out. The tranche_target_device for this tranche will be the secondary device and will not match the main_device.

Some compositors may support switching their rendering device at runtime or changing their rendering device depending on the surface. When the rendering device changes for a surface, such compositors may re-send the feedback parameters with a different main_device. However there is a risk that clients don't support switching their device at runtime and continue using the previous device. For this reason, compositors should always have a fallback rendering device that they initially send as main_device, such that these clients use said fallback device.

Compositors should not change the main_device on-the-fly when explicit modifiers are not supported, because there's a risk of importing buffers with an implicit non-linear modifier as a linear buffer, resulting in misinterpreted buffer contents.

Compositors should not send feedback parameters if they don't have a fallback path. For instance, compositors shouldn't send a format/modifier supported for direct scan-out but not supported by the rendering API for texturing.

Compositors can decide to use multiple tranches to describe the allocation parameters optimized for texturing. For example, if there are formats which have a fast texturing path and formats which have a slower texturing path, the compositor can decide to expose two separate tranches.

Compositors can decide to use intermediate tranches to describe code-paths slower than direct scan-out but faster than texturing. For instance, a compositor could insert an intermediate tranche if it's possible to use a mem2mem device to convert buffers to be able to use scan-out.

dev_t encoding

The protocol carries dev_t values on the wire using arrays. A compositor written in C can encode the values as follows:

.. code-block:: c

struct stat drm_node_stat;
struct wl_array dev_array = {
    .size = sizeof(drm_node_stat.st_rdev),
    .data = &drm_node_stat.st_rdev,
};

A client can decode the values as follows:

.. code-block:: c

dev_t dev;
assert(dev_array->size == sizeof(dev));
memcpy(&dev, dev_array->data, sizeof(dev));

Because two DRM nodes can refer to the same DRM device while having different dev_t values, clients should use drmDevicesEqual to compare two devices.

format_table encoding

The format_table event carries a file descriptor containing a list of format + modifier pairs. The list is an array of pairs which can be accessed with this C structure definition:

.. code-block:: c

struct dmabuf_format_modifier {
    uint32_t format;
    uint32_t pad; /* unused */
    uint64_t modifier;
};

Integration with other APIs

• libdrm: drmGetDeviceFromDevId returns a drmDevice from a device ID.
• EGL: the EGL_EXT_device_drm_render_node_ extension may be used to query the DRM device render node used by a given EGL display. When unavailable, the older EGL_EXT_device_drm_ extension may be used as a fallback.
• Vulkan: the VK_EXT_physical_device_drm_ extension may be used to query the DRM device used by a given VkPhysicalDevice.

.. _EGL_EXT_device_drm: https://www.khronos.org/registry/EGL/extensions/EXT/EGL_EXT_device_drm.txt .. _EGL_EXT_device_drm_render_node: https://www.khronos.org/registry/EGL/extensions/EXT/EGL_EXT_device_drm_render_node.txt .. _VK_EXT_physical_device_drm: https://www.khronos.org/registry/vulkan/specs/1.2-extensions/man/html/VK_EXT_physical_device_drm.html