On the Samsung Galaxy Book4 Edge every since I am getting intermittent
mdt initialization errors from qcpas(4) looking like

	qcpas0: failed to shutdown lite firmware
	qcpas0: failed to receive ready signal
	qcpas0: failed to boot coprocessor

leaving me without the qcpas(4) HW sensors (they anyway ship no data
on the Samsung currently, but that's a different story I want to tackle
next).

Implementing a retry loop will help to overcome this situation:

	qcpas0: failed to shutdown lite firmware
	qcpas0: failed to receive ready signal
	qcpas0: coprocessor boot attempt 2
	qcpas0: failed to receive ready signal
	qcpas0: coprocessor boot attempt 3

Feedback, regression testers, OKs?


Index: sys/dev/fdt/qcpas.c
===================================================================
RCS file: /cvs/src/sys/dev/fdt/qcpas.c,v
diff -u -p -u -p -r1.12 qcpas.c
--- sys/dev/fdt/qcpas.c	1 Aug 2025 08:55:09 -0000	1.12
+++ sys/dev/fdt/qcpas.c	16 May 2026 15:50:14 -0000
@@ -211,7 +211,7 @@ qcpas_mountroot(struct device *self)
 	char fwname[128];
 	size_t fwlen, dtb_fwlen;
 	u_char *fw, *dtb_fw;
-	int node, ret;
+	int i, node, ret;
 	int error;
 
 	if (qcpas_map_memory(sc) != 0)
@@ -272,7 +272,19 @@ qcpas_mountroot(struct device *self)
 		free(dtb_fw, M_DEVBUF, dtb_fwlen);
 	}
 
-	ret = qcpas_mdt_init(sc, sc->sc_pas_id, fw, fwlen);
+	/*
+	 * Coprocessor boot is intermittent on some platforms
+	 * (e.g. Samsung Galaxy Book4 Edge): all of the SCM calls
+	 * succeed but the firmware sometimes silently dies before
+	 * signalling ready.  Retry up to a few times -- when it
+	 * works, the ready signal arrives within a few hundred ms.
+	 */
+	for (ret = -1, i = 0; ret != 0 && i < 3; i++) {
+		if (i > 0)
+			printf("%s: coprocessor boot attempt %d\n",
+			    sc->sc_dev.dv_xname, i + 1);
+		ret = qcpas_mdt_init(sc, sc->sc_pas_id, fw, fwlen);
+	}
 	free(fw, M_DEVBUF, fwlen);
 	if (ret != 0) {
 		printf("%s: failed to boot coprocessor\n",
@@ -467,6 +479,7 @@ qcpas_mdt_init(struct qcpas_softc *sc, i
 	    QCPAS_DMA_DVA(sc->sc_metadata[idx])) != 0) {
 		printf("%s: init image failed\n", sc->sc_dev.dv_xname);
 		qcpas_dmamem_free(sc, sc->sc_metadata[idx]);
+		sc->sc_metadata[idx] = NULL;
 		return EINVAL;
 	}
 
@@ -474,6 +487,7 @@ qcpas_mdt_init(struct qcpas_softc *sc, i
 	    sc->sc_mem_phys[idx], maxpa - minpa) != 0) {
 		printf("%s: mem setup failed\n", sc->sc_dev.dv_xname);
 		qcpas_dmamem_free(sc, sc->sc_metadata[idx]);
+		sc->sc_metadata[idx] = NULL;
 		return EINVAL;
 	}
 
@@ -509,6 +523,7 @@ qcpas_mdt_init(struct qcpas_softc *sc, i
 	if (qcscm_pas_auth_and_reset(pas_id) != 0) {
 		printf("%s: auth and reset failed\n", sc->sc_dev.dv_xname);
 		qcpas_dmamem_free(sc, sc->sc_metadata[idx]);
+		sc->sc_metadata[idx] = NULL;
 		return EINVAL;
 	}
 
@@ -519,6 +534,14 @@ qcpas_mdt_init(struct qcpas_softc *sc, i
 	if (error) {
 		printf("%s: failed to receive ready signal\n",
 		    sc->sc_dev.dv_xname);
+		/*
+		 * The coprocessor was started but never signalled ready.
+		 * Shut it down and release the metadata buffer so the
+		 * caller can retry from scratch.
+		 */
+		qcscm_pas_shutdown(pas_id);
+		qcpas_dmamem_free(sc, sc->sc_metadata[idx]);
+		sc->sc_metadata[idx] = NULL;
 		return error;
 	}