D825 - An export's colour target is a location, bound by SPI_SHADER_COL_FORMAT

Status: decided Date: 2026-10-13

A translated exp mrtK stores to the fragment module's vec4 output at Location K. Every fragment module declares location zero; it declares another location only when one of its exports names that target. This replaces D553's refusal of every target but mrt0.

Which colour buffer location K is comes from the registers the guest wrote. It is the Kth colour buffer whose four-bit SPI_SHADER_COL_FORMAT field is not SPI_SHADER_ZERO: a buffer with no format takes no export number. The pipeline lists those buffers in that order as the render pass's colour attachments. Colour buffer n's registers are read by the decoders written for colour buffer zero, through a view that moves them onto buffer zero's addresses:

• CB_COLORn_BASE to DCC_BASE are fifteen dwords on per buffer.
• Each *_EXT, ATTRIB2 and ATTRIB3 is one dword on.
• CB_BLENDn_CONTROL is one dword on.
• CB_TARGET_MASK and CB_SHADER_MASK are shifted four bits per buffer.

The first attachment keeps D714's write-back, deferred to the flip where it was. Each further attachment is seeded from guest memory before its submission is drawn and written back after it.

A draw is refused, and counted, when its pixel shader writes channels to a target past the colour buffers the submission binds.

Why: the compaction is published. ACO skips a ZERO slot (aco_select_ps_epilog.cpp:88-91) and numbers each export by the exports before it (:428-447). The register strides are published in gfx103.json. So which attachment a target selects is no longer the unknown D553 refused it for. oops-gl's second draw buffer exports mrt0 and mrt1 and binds CB_COLOR1, and both GL probes stopped there.

Rejected:

• Taking target K as colour buffer K: right only while no lower buffer is unformatted, so it draws a plausible wrong frame the first time one is.
• Writing every attachment back at the flip, as the first is: the deferred write-back holds one frame per target (D714, D801), and the further attachments are what a second draw buffer reads back within the same frame.
• Declaring all eight outputs in every fragment module: a module would then claim exports it never makes.