Mesa (master): ac/nir: Remove stale TODO

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Module: Mesa
Branch: master
Commit: 9d93d2a4049c0635e75c8eff85c8be7603599340
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=9d93d2a4049c0635e75c8eff85c8be7603599340

Author: Connor Abbott <cwabbott0 at gmail.com>
Date:   Wed Jun  5 16:54:24 2019 +0200

ac/nir: Remove stale TODO

While we're here, copy the comment explaining this from radeonsi.

Reviewed-by: Samuel Pitoiset <samuel.pitoiset at gmail.com>

---

 src/amd/common/ac_nir_to_llvm.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/src/amd/common/ac_nir_to_llvm.c b/src/amd/common/ac_nir_to_llvm.c
index dd673cf5ea0..360e139bf5f 100644
--- a/src/amd/common/ac_nir_to_llvm.c
+++ b/src/amd/common/ac_nir_to_llvm.c
@@ -3895,7 +3895,13 @@ static void visit_tex(struct ac_nir_context *ctx, nir_tex_instr *instr)
 		args.offset = NULL;
 	}
 
-	/* TODO TG4 support */
+	/* DMASK was repurposed for GATHER4. 4 components are always
+	 * returned and DMASK works like a swizzle - it selects
+	 * the component to fetch. The only valid DMASK values are
+	 * 1=red, 2=green, 4=blue, 8=alpha. (e.g. 1 returns
+	 * (red,red,red,red) etc.) The ISA document doesn't mention
+	 * this.
+	 */
 	args.dmask = 0xf;
 	if (instr->op == nir_texop_tg4) {
 		if (instr->is_shadow)