• Lyude Paul's avatar
    drm/nouveau/kms/nvd9-: Add CRC support · 12885ecb
    Lyude Paul authored
    This introduces support for CRC readback on gf119+, using the
    documentation generously provided to us by Nvidia:
    
    https://github.com/NVIDIA/open-gpu-doc/blob/master/Display-CRC/display-crc.txt
    
    We expose all available CRC sources. SF, SOR, PIOR, and DAC are exposed
    through a single set of "outp" sources: outp-active/auto for a CRC of
    the scanout region, outp-complete for a CRC of both the scanout and
    blanking/sync region combined, and outp-inactive for a CRC of only the
    blanking/sync region. For each source, nouveau selects the appropriate
    tap point based on the output path in use. We also expose an "rg"
    source, which allows for capturing CRCs of the scanout raster before
    it's encoded into a video signal in the output path. This tap point is
    referred to as the raster generator.
    
    Note that while there's some other neat features that can be used with
    CRC capture on nvidia hardware, like capturing from two CRC sources
    simultaneously, I couldn't see any usecase for them and did not
    implement them.
    
    Nvidia only allows for accessing CRCs through a shared DMA region that
    we program through the core EVO/NvDisplay channel which is referred to
    as the notifier context. The notifier context is limited to either 255
    (for Fermi-Pascal) or 2047 (Volta+) entries to store CRCs in, and
    unfortunately the hardware simply drops CRCs and reports an overflow
    once all available entries in the notifier context are filled.
    
    Since the DRM CRC API and igt-gpu-tools don't expect there to be a limit
    on how many CRCs can be captured, we work around this in nouveau by
    allocating two separate notifier contexts for each head instead of one.
    We schedule a vblank worker ahead of time so that once we start getting
    close to filling up all of the available entries in the notifier
    context, we can swap the currently used notifier context out with
    another pre-prepared notifier context in a manner similar to page
    flipping.
    
    Unfortunately, the hardware only allows us to this by flushing two
    separate updates on the core channel: one to release the current
    notifier context handle, and one to program the next notifier context's
    handle. When the hardware processes the first update, the CRC for the
    current frame is lost. However, the second update can be flushed
    immediately without waiting for the first to complete so that CRC
    generation resumes on the next frame. According to Nvidia's hardware
    engineers, there isn't any cleaner way of flipping notifier contexts
    that would avoid this.
    
    Since using vblank workers to swap out the notifier context will ensure
    we can usually flush both updates to hardware within the timespan of a
    single frame, we can also ensure that there will only be exactly one
    frame lost between the first and second update being executed by the
    hardware. This gives us the guarantee that we're always correctly
    matching each CRC entry with it's respective frame even after a context
    flip. And since IGT will retrieve the CRC entry for a frame by waiting
    until it receives a CRC for any subsequent frames, this doesn't cause an
    issue with any tests and is much simpler than trying to change the
    current DRM API to accommodate.
    
    In order to facilitate testing of correct handling of this limitation,
    we also expose a debugfs interface to manually control the threshold for
    when we start trying to flip the notifier context. We will use this in
    igt to trigger a context flip for testing purposes without needing to
    wait for the notifier to completely fill up. This threshold is reset
    to the default value set by nouveau after each capture, and is exposed
    in a separate folder within each CRTC's debugfs directory labelled
    "nv_crc".
    
    Changes since v1:
    * Forgot to finish saving crc.h before saving, whoops. This just adds
      some corrections to the empty function declarations that we use if
      CONFIG_DEBUG_FS isn't enabled.
    Changes since v2:
    * Don't check return code from debugfs_create_dir() or
      debugfs_create_file() - Greg K-H
    Changes since v3:
      (no functional changes)
    * Fix SPDX license identifiers (checkpatch)
    * s/uint32_t/u32/ (checkpatch)
    * Fix indenting in switch cases (checkpatch)
    Changes since v4:
    * Remove unneeded param changes with nv50_head_flush_clr/set
    * Rebase
    Changes since v5:
    * Remove set but unused variable (outp) in nv50_crc_atomic_check() -
      Kbuild bot
    Signed-off-by: default avatarLyude Paul <lyude@redhat.com>
    Reviewed-by: default avatarBen Skeggs <bskeggs@redhat.com>
    Acked-by: default avatarDave Airlie <airlied@gmail.com>
    Link: https://patchwork.freedesktop.org/patch/msgid/20200627194657.156514-10-lyude@redhat.com
    12885ecb
crtc.c 42.5 KB