drm/nouveau/i2c/aux/g94-: retry transactions after hw reports an error

This fixes (works around?) link training failures seen on (at least)
the Lenovo P50's internal panel.

It's also an important fix on the same system for MST support on the
dock.  Sometimes, right after receiving an IRQ from the sink, there's
an error bit (SINKSTAT_ERR) set in the DPAUX registers before we've
even attempted a transaction.

v2. Fixed regression on passive DP->DVI adapters.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nvkm/subdev/i2c/auxg94.c

@@ -79,7 +79,7 @@ g94_i2c_aux_xfer(struct nvkm_i2c_aux *obj, bool retry,
 	struct g94_i2c_aux *aux = g94_i2c_aux(obj);
 	struct nvkm_device *device = aux->base.pad->i2c->subdev.device;
 	const u32 base = aux->ch * 0x50;
-	u32 ctrl, stat, timeout, retries;
+	u32 ctrl, stat, timeout, retries = 0;
 	u32 xbuf[4] = {};
 	int ret, i;
 
@@ -111,7 +111,7 @@ g94_i2c_aux_xfer(struct nvkm_i2c_aux *obj, bool retry,
 	nvkm_wr32(device, 0x00e4e0 + base, addr);
 
 	/* (maybe) retry transaction a number of times on failure... */
-	for (retries = 0; !ret && retries < 32; retries++) {
+	do {
 		/* reset, and delay a while if this is a retry */
 		nvkm_wr32(device, 0x00e4e4 + base, 0x80000000 | ctrl);
 		nvkm_wr32(device, 0x00e4e4 + base, 0x00000000 | ctrl);
@@ -131,20 +131,20 @@ g94_i2c_aux_xfer(struct nvkm_i2c_aux *obj, bool retry,
 				goto out;
 			}
 		} while (ctrl & 0x00010000);
-		ret = 1;
+		ret = 0;
 
 		/* read status, and check if transaction completed ok */
 		stat = nvkm_mask(device, 0x00e4e8 + base, 0, 0);
 		if ((stat & 0x000f0000) == 0x00080000 ||
 		    (stat & 0x000f0000) == 0x00020000)
-			ret = retry ? 0 : 1;
+			ret = 1;
 		if ((stat & 0x00000100))
 			ret = -ETIMEDOUT;
 		if ((stat & 0x00000e00))
 			ret = -EIO;
 
 		AUX_TRACE(&aux->base, "%02d %08x %08x", retries, ctrl, stat);
-	}
+	} while (ret && retry && retries++ < 32);
 
 	if (type & 1) {
 		for (i = 0; i < 16; i += 4) {

drivers/gpu/drm/nouveau/nvkm/subdev/i2c/auxgm200.c

@@ -79,7 +79,7 @@ gm200_i2c_aux_xfer(struct nvkm_i2c_aux *obj, bool retry,
 	struct gm200_i2c_aux *aux = gm200_i2c_aux(obj);
 	struct nvkm_device *device = aux->base.pad->i2c->subdev.device;
 	const u32 base = aux->ch * 0x50;
-	u32 ctrl, stat, timeout, retries;
+	u32 ctrl, stat, timeout, retries = 0;
 	u32 xbuf[4] = {};
 	int ret, i;
 
@@ -111,7 +111,7 @@ gm200_i2c_aux_xfer(struct nvkm_i2c_aux *obj, bool retry,
 	nvkm_wr32(device, 0x00d950 + base, addr);
 
 	/* (maybe) retry transaction a number of times on failure... */
-	for (retries = 0; !ret && retries < 32; retries++) {
+	do {
 		/* reset, and delay a while if this is a retry */
 		nvkm_wr32(device, 0x00d954 + base, 0x80000000 | ctrl);
 		nvkm_wr32(device, 0x00d954 + base, 0x00000000 | ctrl);
@@ -131,20 +131,20 @@ gm200_i2c_aux_xfer(struct nvkm_i2c_aux *obj, bool retry,
 				goto out;
 			}
 		} while (ctrl & 0x00010000);
-		ret = 1;
+		ret = 0;
 
 		/* read status, and check if transaction completed ok */
 		stat = nvkm_mask(device, 0x00d958 + base, 0, 0);
 		if ((stat & 0x000f0000) == 0x00080000 ||
 		    (stat & 0x000f0000) == 0x00020000)
-			ret = retry ? 0 : 1;
+			ret = 1;
 		if ((stat & 0x00000100))
 			ret = -ETIMEDOUT;
 		if ((stat & 0x00000e00))
 			ret = -EIO;
 
 		AUX_TRACE(&aux->base, "%02d %08x %08x", retries, ctrl, stat);
-	}
+	} while (ret && retry && retries++ < 32);
 
 	if (type & 1) {
 		for (i = 0; i < 16; i += 4) {