Documentation/gpu: Document how to narrow down display issues

The amdgpu driver is composed of multiple components, each of which can
be a source of some specific problem that the user/developer can see.
This commit introduces steps to narrow down and collect display
information.

Cc: Leo Li <[e-mail]>
Cc: Aurabindo Pillai <[e-mail]>
Cc: Hamza Mahfooz <[e-mail]>
Cc: Harry Wentland <[e-mail]>
Cc: Mario Limonciello <[e-mail]>
Cc: Christian Konig <[e-mail]>
Cc: Alex Deucher <[e-mail]>
Signed-off-by: Rodrigo Siqueira <[e-mail]>
Reviewed-by: Mario Limonciello <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

Documentation/gpu/amdgpu/display/dc-debug.rst

···
 Display Core Debug tools
 ========================
 
+In this section, you will find helpful information on debugging the amdgpu
+driver from the display perspective. This page introduces debug mechanisms and
+procedures to help you identify if some issues are related to display code.
+
+Narrow down display issues
+==========================
+
+Since the display is the driver's visual component, it is common to see users
+reporting issues as a display when another component causes the problem. This
+section equips users to determine if a specific issue was caused by the display
+component or another part of the driver.
+
+DC dmesg important messages
+---------------------------
+
+The dmesg log is the first source of information to be checked, and amdgpu
+takes advantage of this feature by logging some valuable information. When
+looking for the issues associated with amdgpu, remember that each component of
+the driver (e.g., smu, PSP, dm, etc.) is loaded one by one, and this
+information can be found in the dmesg log. In this sense, look for the part of
+the log that looks like the below log snippet::
+
+  [    4.254295] [drm] initializing kernel modesetting (IP DISCOVERY 0x1002:0x744C 0x1002:0x0E3B 0xC8).
+  [    4.254718] [drm] register mmio base: 0xFCB00000
+  [    4.254918] [drm] register mmio size: 1048576
+  [    4.260095] [drm] add ip block number 0 <soc21_common>
+  [    4.260318] [drm] add ip block number 1 <gmc_v11_0>
+  [    4.260510] [drm] add ip block number 2 <ih_v6_0>
+  [    4.260696] [drm] add ip block number 3 <psp>
+  [    4.260878] [drm] add ip block number 4 <smu>
+  [    4.261057] [drm] add ip block number 5 <dm>
+  [    4.261231] [drm] add ip block number 6 <gfx_v11_0>
+  [    4.261402] [drm] add ip block number 7 <sdma_v6_0>
+  [    4.261568] [drm] add ip block number 8 <vcn_v4_0>
+  [    4.261729] [drm] add ip block number 9 <jpeg_v4_0>
+  [    4.261887] [drm] add ip block number 10 <mes_v11_0>
+
+From the above example, you can see the line that reports that `<dm>`,
+(**Display Manager**), was loaded, which means that display can be part of the
+issue. If you do not see that line, something else might have failed before
+amdgpu loads the display component, indicating that we don't have a
+display issue.
+
+After you identified that the DM was loaded correctly, you can check for the
+display version of the hardware in use, which can be retrieved from the dmesg
+log with the command::
+
+  dmesg | grep -i 'display core'
+
+This command shows a message that looks like this::
+
+  [    4.655828] [drm] Display Core v3.2.285 initialized on DCN 3.2
+
+This message has two key pieces of information:
+
+* **The DC version (e.g., v3.2.285)**: Display developers release a new DC version
+  every week, and this information can be advantageous in a situation where a
+  user/developer must find a good point versus a bad point based on a tested
+  version of the display code. Remember from page :ref:`Display Core <amdgpu-display-core>`,
+  that every week the new patches for display are heavily tested with IGT and
+  manual tests.
+* **The DCN version (e.g., DCN 3.2)**: The DCN block is associated with the
+  hardware generation, and the DCN version conveys the hardware generation that
+  the driver is currently running. This information helps to narrow down the
+  code debug area since each DCN version has its files in the DC folder per DCN
+  component (from the example, the developer might want to focus on
+  files/folders/functions/structs with the dcn32 label might be executed).
+  However, keep in mind that DC reuses code across different DCN versions; for
+  example, it is expected to have some callbacks set in one DCN that are the same
+  as those from another DCN. In summary, use the DCN version just as a guide.
+
+From the dmesg file, it is also possible to get the ATOM bios code by using::
+
+  dmesg  | grep -i 'ATOM BIOS'
+
+Which generates an output that looks like this::
+
+  [    4.274534] amdgpu: ATOM BIOS: 113-D7020100-102
+
+This type of information is useful to be reported.
+
+Avoid loading display core
+--------------------------
+
+Sometimes, it might be hard to figure out which part of the driver is causing
+the issue; if you suspect that the display is not part of the problem and your
+bug scenario is simple (e.g., some desktop configuration) you can try to remove
+the display component from the equation. First, you need to identify `dm` ID
+from the dmesg log; for example, search for the following log::
+
+  [    4.254295] [drm] initializing kernel modesetting (IP DISCOVERY 0x1002:0x744C 0x1002:0x0E3B 0xC8).
+  [..]
+  [    4.260095] [drm] add ip block number 0 <soc21_common>
+  [    4.260318] [drm] add ip block number 1 <gmc_v11_0>
+  [..]
+  [    4.261057] [drm] add ip block number 5 <dm>
+
+Notice from the above example that the `dm` id is 5 for this specific hardware.
+Next, you need to run the following binary operation to identify the IP block
+mask::
+
+  0xffffffff & ~(1 << [DM ID])
+
+From our example the IP mask is::
+
+ 0xffffffff & ~(1 << 5) = 0xffffffdf
+
+Finally, to disable DC, you just need to set the below parameter in your
+bootloader::
+
+ amdgpu.ip_block_mask = 0xffffffdf
+
+If you can boot your system with the DC disabled and still see the issue, it
+means you can rule DC out of the equation. However, if the bug disappears, you
+still need to consider the DC part of the problem and keep narrowing down the
+issue. In some scenarios, disabling DC is impossible since it might be
+necessary to use the display component to reproduce the issue (e.g., play a
+game).
+
+**Note: This will probably lead to the absence of a display output.**
+
+Display flickering
+------------------
+
+Display flickering might have multiple causes; one is the lack of proper power
+to the GPU or problems in the DPM switches. A good first generic verification
+is to set the GPU to use high voltage::
+
+   bash -c "echo high > /sys/class/drm/card0/device/power_dpm_force_performance_level"
+
+The above command sets the GPU/APU to use the maximum power allowed which
+disables DPM switches. If forcing DPM levels high does not fix the issue, it
+is less likely that the issue is related to power management. If the issue
+disappears, there is a good chance that other components might be involved, and
+the display should not be ignored since this could be a DPM issues. From the
+display side, if the power increase fixes the issue, it is worth debugging the
+clock configuration and the pipe split police used in the specific
+configuration.
+
+Display artifacts
+-----------------
+
+Users may see some screen artifacts that can be categorized into two different
+types: localized artifacts and general artifacts. The localized artifacts
+happen in some specific areas, such as around the UI window corners; if you see
+this type of issue, there is a considerable chance that you have a userspace
+problem, likely Mesa or similar. The general artifacts usually happen on the
+entire screen. They might be caused by a misconfiguration at the driver level
+of the display parameters, but the userspace might also cause this issue. One
+way to identify the source of the problem is to take a screenshot or make a
+desktop video capture when the problem happens; after checking the
+screenshot/video recording, if you don't see any of the artifacts, it means
+that the issue is likely on the the driver side. If you can still see the
+problem in the data collected, it is an issue that probably happened during
+rendering, and the display code just got the framebuffer already corrupted.
+
+Disabling/Enabling specific features
+====================================
+
+DC has a struct named `dc_debug_options`, which is statically initialized by
+all DCE/DCN components based on the specific hardware characteristic. This
+structure usually facilitates the bring-up phase since developers can start
+with many disabled features and enable them individually. This is also an
+important debug feature since users can change it when debugging specific
+issues.
+
+For example, dGPU users sometimes see a problem where a horizontal fillet of
+flickering happens in some specific part of the screen. This could be an
+indication of Sub-Viewport issues; after the users identified the target DCN,
+they can set the `force_disable_subvp` field to true in the statically
+initialized version of `dc_debug_options` to see if the issue gets fixed. Along
+the same lines, users/developers can also try to turn off `fams2_config` and
+`enable_single_display_2to1_odm_policy`. In summary, the `dc_debug_options` is
+an interesting form for identifying the problem.
+
 DC Visual Confirmation
 ======================
 
···
 
 When reporting a bug related to DC, consider attaching this log before and
 after you reproduce the bug.
+
+Collect Firmware information
+============================
+
+When reporting issues, it is important to have the firmware information since
+it can be helpful for debugging purposes. To get all the firmware information,
+use the command::
+
+  cat /sys/kernel/debug/dri/0/amdgpu_firmware_info
+
+From the display perspective, pay attention to the firmware of the DMCU and
+DMCUB.
 
 DMUB Firmware Debug
 ===================