[RFC PATCH RESEND v4 0/6] drm/display: dp: add new DPCD access functions

[Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>]

Existing DPCD access functions return an error code or the number of
bytes being read / write in case of partial access. However a lot of
drivers either (incorrectly) ignore partial access or mishandle error
codes. In other cases this results in a boilerplate code which compares
returned value with the size.

As suggested by Jani implement new set of DPCD access helpers, which
ignore partial access, always return 0 or an error code. Implement
new helpers using existing functions to ensure backwards compatibility
and to assess necessity to handle incomplete reads on a global scale.
Currently only one possible place has been identified, dp-aux-dev, which
needs to handle possible holes in DPCD.

This series targets only the DRM helpers code. If the approach is found
to be acceptable, each of the drivers should be converted on its own.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
---
Changes in v4:
- Actually dropped the dp-aux-dev patch (Lyude).
- Added two missing full stops in linuxdoc (Lyude).
- Link to v3: https://lore.kernel.org/r/20250307-drm-rework-dpcd-access-v3-0-9044a3a868ee@linaro.org

Changes in v3:
- Fixed cover letter (Jani)
- Added intel-gfx and intel-xe to get the series CI-tested (Jani)
- Link to v2: https://lore.kernel.org/r/20250301-drm-rework-dpcd-access-v2-0-4d92602fc7cd@linaro.org

Changes in v2:
- Reimplemented new helpers using old ones (Lyude)
- Reworked the drm_dp_dpcd_read_link_status() patch (Lyude)
- Dropped the dp-aux-dev patch (Jani)
- Link to v1: https://lore.kernel.org/r/20250117-drm-rework-dpcd-access-v1-0-7fc020e04dbc@linaro.org

---
Dmitry Baryshkov (6):
      drm/display: dp: implement new access helpers
      drm/display: dp: change drm_dp_dpcd_read_link_status() return value
      drm/display: dp: use new DCPD access helpers
      drm/display: dp-cec: use new DCPD access helpers
      drm/display: dp-mst-topology: use new DCPD access helpers
      drm/display: dp-tunnel: use new DCPD access helpers

 drivers/gpu/drm/amd/amdgpu/atombios_dp.c           |   8 +-
 .../gpu/drm/bridge/cadence/cdns-mhdp8546-core.c    |   2 +-
 drivers/gpu/drm/display/drm_dp_cec.c               |  37 ++-
 drivers/gpu/drm/display/drm_dp_helper.c            | 307 +++++++++------------
 drivers/gpu/drm/display/drm_dp_mst_topology.c      | 105 ++++---
 drivers/gpu/drm/display/drm_dp_tunnel.c            |  20 +-
 drivers/gpu/drm/hisilicon/hibmc/dp/dp_link.c       |   4 +-
 drivers/gpu/drm/msm/dp/dp_ctrl.c                   |  24 +-
 drivers/gpu/drm/msm/dp/dp_link.c                   |  18 +-
 drivers/gpu/drm/radeon/atombios_dp.c               |   8 +-
 include/drm/display/drm_dp_helper.h                |  92 +++++-
 11 files changed, 317 insertions(+), 308 deletions(-)
---
base-commit: b0894e40afe2bd05d1fda68cc364665ac2b00e09
change-id: 20241231-drm-rework-dpcd-access-b0fc2e47d613

Best regards,
-- 
Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>