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diff --git a/Documentation/gpu/amdgpu/display/dcn-overview.rst b/Documentation/gpu/amdgpu/display/dcn-overview.rst new file mode 100644 index 000000000..9fea65004 --- /dev/null +++ b/Documentation/gpu/amdgpu/display/dcn-overview.rst @@ -0,0 +1,230 @@ +======================= +Display Core Next (DCN) +======================= + +To equip our readers with the basic knowledge of how AMD Display Core Next +(DCN) works, we need to start with an overview of the hardware pipeline. Below +you can see a picture that provides a DCN overview, keep in mind that this is a +generic diagram, and we have variations per ASIC. + +.. kernel-figure:: dc_pipeline_overview.svg + +Based on this diagram, we can pass through each block and briefly describe +them: + +* **Display Controller Hub (DCHUB)**: This is the gateway between the Scalable + Data Port (SDP) and DCN. This component has multiple features, such as memory + arbitration, rotation, and cursor manipulation. + +* **Display Pipe and Plane (DPP)**: This block provides pre-blend pixel + processing such as color space conversion, linearization of pixel data, tone + mapping, and gamut mapping. + +* **Multiple Pipe/Plane Combined (MPC)**: This component performs blending of + multiple planes, using global or per-pixel alpha. + +* **Output Pixel Processing (OPP)**: Process and format pixels to be sent to + the display. + +* **Output Pipe Timing Combiner (OPTC)**: It generates time output to combine + streams or divide capabilities. CRC values are generated in this block. + +* **Display Output (DIO)**: Codify the output to the display connected to our + GPU. + +* **Display Writeback (DWB)**: It provides the ability to write the output of + the display pipe back to memory as video frames. + +* **Multi-Media HUB (MMHUBBUB)**: Memory controller interface for DMCUB and DWB + (Note that DWB is not hooked yet). + +* **DCN Management Unit (DMU)**: It provides registers with access control and + interrupts the controller to the SOC host interrupt unit. This block includes + the Display Micro-Controller Unit - version B (DMCUB), which is handled via + firmware. + +* **DCN Clock Generator Block (DCCG)**: It provides the clocks and resets + for all of the display controller clock domains. + +* **Azalia (AZ)**: Audio engine. + +The above diagram is an architecture generalization of DCN, which means that +every ASIC has variations around this base model. Notice that the display +pipeline is connected to the Scalable Data Port (SDP) via DCHUB; you can see +the SDP as the element from our Data Fabric that feeds the display pipe. + +Always approach the DCN architecture as something flexible that can be +configured and reconfigured in multiple ways; in other words, each block can be +setup or ignored accordingly with userspace demands. For example, if we +want to drive an 8k@60Hz with a DSC enabled, our DCN may require 4 DPP and 2 +OPP. It is DC's responsibility to drive the best configuration for each +specific scenario. Orchestrate all of these components together requires a +sophisticated communication interface which is highlighted in the diagram by +the edges that connect each block; from the chart, each connection between +these blocks represents: + +1. Pixel data interface (red): Represents the pixel data flow; +2. Global sync signals (green): It is a set of synchronization signals composed + by VStartup, VUpdate, and VReady; +3. Config interface: Responsible to configure blocks; +4. Sideband signals: All other signals that do not fit the previous one. + +These signals are essential and play an important role in DCN. Nevertheless, +the Global Sync deserves an extra level of detail described in the next +section. + +All of these components are represented by a data structure named dc_state. +From DCHUB to MPC, we have a representation called dc_plane; from MPC to OPTC, +we have dc_stream, and the output (DIO) is handled by dc_link. Keep in mind +that HUBP accesses a surface using a specific format read from memory, and our +dc_plane should work to convert all pixels in the plane to something that can +be sent to the display via dc_stream and dc_link. + +Front End and Back End +---------------------- + +Display pipeline can be broken down into two components that are usually +referred as **Front End (FE)** and **Back End (BE)**, where FE consists of: + +* DCHUB (Mainly referring to a subcomponent named HUBP) +* DPP +* MPC + +On the other hand, BE consist of + +* OPP +* OPTC +* DIO (DP/HDMI stream encoder and link encoder) + +OPP and OPTC are two joining blocks between FE and BE. On a side note, this is +a one-to-one mapping of the link encoder to PHY, but we can configure the DCN +to choose which link encoder to connect to which PHY. FE's main responsibility +is to change, blend and compose pixel data, while BE's job is to frame a +generic pixel stream to a specific display's pixel stream. + +Data Flow +--------- + +Initially, data is passed in from VRAM through Data Fabric (DF) in native pixel +formats. Such data format stays through till HUBP in DCHUB, where HUBP unpacks +different pixel formats and outputs them to DPP in uniform streams through 4 +channels (1 for alpha + 3 for colors). + +The Converter and Cursor (CNVC) in DPP would then normalize the data +representation and convert them to a DCN specific floating-point format (i.e., +different from the IEEE floating-point format). In the process, CNVC also +applies a degamma function to transform the data from non-linear to linear +space to relax the floating-point calculations following. Data would stay in +this floating-point format from DPP to OPP. + +Starting OPP, because color transformation and blending have been completed +(i.e alpha can be dropped), and the end sinks do not require the precision and +dynamic range that floating points provide (i.e. all displays are in integer +depth format), bit-depth reduction/dithering would kick in. In OPP, we would +also apply a regamma function to introduce the gamma removed earlier back. +Eventually, we output data in integer format at DIO. + +AMD Hardware Pipeline +--------------------- + +When discussing graphics on Linux, the **pipeline** term can sometimes be +overloaded with multiple meanings, so it is important to define what we mean +when we say **pipeline**. In the DCN driver, we use the term **hardware +pipeline** or **pipeline** or just **pipe** as an abstraction to indicate a +sequence of DCN blocks instantiated to address some specific configuration. DC +core treats DCN blocks as individual resources, meaning we can build a pipeline +by taking resources for all individual hardware blocks to compose one pipeline. +In actuality, we can't connect an arbitrary block from one pipe to a block from +another pipe; they are routed linearly, except for DSC, which can be +arbitrarily assigned as needed. We have this pipeline concept for trying to +optimize bandwidth utilization. + +.. kernel-figure:: pipeline_4k_no_split.svg + +Additionally, let's take a look at parts of the DTN log (see +'Documentation/gpu/amdgpu/display/dc-debug.rst' for more information) since +this log can help us to see part of this pipeline behavior in real-time:: + + HUBP: format addr_hi width height ... + [ 0]: 8h 81h 3840 2160 + [ 1]: 0h 0h 0 0 + [ 2]: 0h 0h 0 0 + [ 3]: 0h 0h 0 0 + [ 4]: 0h 0h 0 0 + ... + MPCC: OPP DPP ... + [ 0]: 0h 0h ... + +The first thing to notice from the diagram and DTN log it is the fact that we +have different clock domains for each part of the DCN blocks. In this example, +we have just a single **pipeline** where the data flows from DCHUB to DIO, as +we intuitively expect. Nonetheless, DCN is flexible, as mentioned before, and +we can split this single pipe differently, as described in the below diagram: + +.. kernel-figure:: pipeline_4k_split.svg + +Now, if we inspect the DTN log again we can see some interesting changes:: + + HUBP: format addr_hi width height ... + [ 0]: 8h 81h 1920 2160 ... + ... + [ 4]: 0h 0h 0 0 ... + [ 5]: 8h 81h 1920 2160 ... + ... + MPCC: OPP DPP ... + [ 0]: 0h 0h ... + [ 5]: 0h 5h ... + +From the above example, we now split the display pipeline into two vertical +parts of 1920x2160 (i.e., 3440x2160), and as a result, we could reduce the +clock frequency in the DPP part. This is not only useful for saving power but +also to better handle the required throughput. The idea to keep in mind here is +that the pipe configuration can vary a lot according to the display +configuration, and it is the DML's responsibility to set up all required +configuration parameters for multiple scenarios supported by our hardware. + +Global Sync +----------- + +Many DCN registers are double buffered, most importantly the surface address. +This allows us to update DCN hardware atomically for page flips, as well as +for most other updates that don't require enabling or disabling of new pipes. + +(Note: There are many scenarios when DC will decide to reserve extra pipes +in order to support outputs that need a very high pixel clock, or for +power saving purposes.) + +These atomic register updates are driven by global sync signals in DCN. In +order to understand how atomic updates interact with DCN hardware, and how DCN +signals page flip and vblank events it is helpful to understand how global sync +is programmed. + +Global sync consists of three signals, VSTARTUP, VUPDATE, and VREADY. These are +calculated by the Display Mode Library - DML (drivers/gpu/drm/amd/display/dc/dml) +based on a large number of parameters and ensure our hardware is able to feed +the DCN pipeline without underflows or hangs in any given system configuration. +The global sync signals always happen during VBlank, are independent from the +VSync signal, and do not overlap each other. + +VUPDATE is the only signal that is of interest to the rest of the driver stack +or userspace clients as it signals the point at which hardware latches to +atomically programmed (i.e. double buffered) registers. Even though it is +independent of the VSync signal we use VUPDATE to signal the VSync event as it +provides the best indication of how atomic commits and hardware interact. + +Since DCN hardware is double-buffered the DC driver is able to program the +hardware at any point during the frame. + +The below picture illustrates the global sync signals: + +.. kernel-figure:: global_sync_vblank.svg + +These signals affect core DCN behavior. Programming them incorrectly will lead +to a number of negative consequences, most of them quite catastrophic. + +The following picture shows how global sync allows for a mailbox style of +updates, i.e. it allows for multiple re-configurations between VUpdate +events where only the last configuration programmed before the VUpdate signal +becomes effective. + +.. kernel-figure:: config_example.svg |