[PATCH v2 0/2] drm/mipi-dbi: display a cropped region of an oversized framebuffer

[Jonathan Frazin <frazinjonathan@gmail.com>]

A drm/tiny MIPI DBI panel can currently only scan out a framebuffer that is exactly panel-sized, always from the origin. This series lets a client allocate a larger framebuffer and choose the displayed region via the plane source rectangle - a crop / pan with no scaling.

Motivation: feeding hardware-decoded video to a small SPI panel. The video decoder emits a fixed frame size; being able to point the panel at a panel-sized window of that buffer avoids a full-frame CPU copy on every flush and lets userspace pan/centre the image.

Patch 1 is the actual fix - mipi_dbi_fb_dirty() addressed the controller in framebuffer coordinates, which is only correct while src_x/src_y are zero. It now subtracts the plane source origin.

Patch 2 raises mode_config.max_width/height (pinned to the panel size) on the six drm/tiny drivers that flush through the shared drm_mipi_dbi_plane_helper_atomic_update(). ili9225 is excluded - it has its own flush path that does not carry the source offset.

Open question for patch 2: the six drivers each set the limits identically in their probe. This could instead be a shared helper (or folded into drm_mipi_dbi_dev_init / the DRM_MIPI_DBI_MODE_CONFIG_* macros) so future drivers get it for free. Happy to respin that way if preferred - the per-driver form is what is shown here because it is the smaller diff and easier to review a first pass.

The min_width/min_height, the fixed mode, the connector and the mode-sized transfer buffer are all unchanged, and drm_mipi_dbi_plane_helper_atomic_check() still enforces DRM_PLANE_NO_SCALING and no repositioning, so the flushed rectangle stays bounded by the panel regardless of the framebuffer dimensions.

Tested on hardware: ILI9341 and ST7789V (through panel-mipi-dbi), both 240x320 - an oversized framebuffer is accepted and a non-zero-offset panel-sized window scans out correctly; a panel-sized framebuffer is unchanged. HX8357D was tested during the downstream review by Dave Stevenson (Cc'd). ili9486, mi0283qt and ili9163 are build-tested only.

Both patches have been carried in the Raspberry Pi kernel (rpi-7.2.y) and in use there: https://github.com/raspberrypi/linux/pull/7589

Applies to current mainline / drm-misc-next; the touched files are identical there.

Changes since v1:
- Patch 1: clamp the damage rectangle to the panel's fixed mode before
  using it, instead of trusting it to already be panel-sized. src_x/src_y
  are the plane source origin truncated to whole pixels, but the
  rectangle from drm_atomic_helper_damage_merged() is clipped against
  that origin's exact 16.16 fixed-point value - when the origin has a
  fractional part, the rectangle's far edge could land up to a pixel past
  where the truncated origin would place the panel's own width/height,
  overflowing the panel-sized tx_buf. Thanks to the automated review for
  catching this.
- Patch 2: unchanged.

Jonathan Frazin (2):
  drm/mipi-dbi: honour the plane source offset when flushing
  drm/tiny: allow a framebuffer larger than the panel on MIPI DBI
    drivers

 drivers/gpu/drm/drm_mipi_dbi.c        | 33 +++++++++++++++++++++++----
 drivers/gpu/drm/tiny/hx8357d.c        |  8 +++++--
 drivers/gpu/drm/tiny/ili9163.c        |  8 +++++--
 drivers/gpu/drm/tiny/ili9341.c        |  8 +++++--
 drivers/gpu/drm/tiny/ili9486.c        |  8 +++++--
 drivers/gpu/drm/tiny/mi0283qt.c       |  8 +++++--
 drivers/gpu/drm/tiny/panel-mipi-dbi.c |  8 +++++--
 7 files changed, 64 insertions(+), 17 deletions(-)

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2.53.0