D794 - A descriptor Mesa calls null with zero selects samples zero

Status: decided Date: 2026-10-11

An image descriptor whose second word is zero and whose DST_SEL_X .. DST_SEL_W are all SQ_SEL_0 binds as the null texture: one texel, zero in every channel. Whatever its other words hold, it is read neither as an address nor as a format.

Two independent groundings agree:

• Mesa treats a descriptor whose second word is zero as null, and answers zero for it (ac_nir_lower_resinfo.c:20-24, handle_null_desc). That word holds the format and the top of the base address, so such a descriptor names no image.
• Each select set to SQ_SEL_0 makes a sample return the constant zero in that channel whatever the texel (gfx10-rsrc.json, SQ_IMG_RSRC_WORD3).

An all-zero descriptor, the null texture before this decision (RADV's memset(dst, 0, size)), is one case of this rule.

Assumed (known_by = assumed): that the hardware fetches nothing it could fault on for such a descriptor. A descriptor with a zero second word and any select other than SQ_SEL_0 stays unbound and refused by name.

Why: PPSA28061, run on the host clock, blends two textures as B + s0 * clamp(A - B) with s0 = 0. Texture A's eight user-data words are 0, 0, 0x800eb0, 0x6000, ...: Mesa's null, with every select zero. The draw was refused, which stopped the submission and the ten behind it.

Rejected:

• Binding any descriptor whose selects are all constants: a descriptor that names an image would then rest on the selects alone, a reading no driver documents.