dmaengine: pl08x: be fair when re-assigning physical channel

Current way we find a waiting virtual channel for the next transfer
at the time one physical channel becomes free is not really fair.

More in details, in case there is more than one channel waiting at a time,
by just going through the arrays of memcpy and slave channels and stopping
as soon as state match waiting state, channels with high indexes can be
penalized.

Whenever dma engine is substantially overloaded so that we constantly
get several channels waiting, channels with highest indexes might not
be served for a substantial time which in the worse case, might hang
task that wait for dma transfer to complete.

This patch makes physical channel re-assignment more fair by storing
time in jiffies when a channel is put in waiting state. Whenever a
physical channel has to be re-assigned, this time is used to select
channel that is waiting for the longest time.
Signed-off-by: Jean-Nicolas Graux <jean-nicolas.graux@st.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Nicolas Guion <nicolas.guion@st.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/amba-pl08x.c

@@ -254,6 +254,7 @@ enum pl08x_dma_chan_state {
  * @slave: whether this channel is a device (slave) or for memcpy
  * @signal: the physical DMA request signal which this channel is using
  * @mux_use: count of descriptors using this DMA request signal setting
+ * @waiting_at: time in jiffies when this channel moved to waiting state
  */
 struct pl08x_dma_chan {
 	struct virt_dma_chan vc;
@@ -267,6 +268,7 @@ struct pl08x_dma_chan {
 	bool slave;
 	int signal;
 	unsigned mux_use;
+	unsigned long waiting_at;
 };
 
 /**
@@ -875,6 +877,7 @@ static void pl08x_phy_alloc_and_start(struct pl08x_dma_chan *plchan)
 	if (!ch) {
 		dev_dbg(&pl08x->adev->dev, "no physical channel available for xfer on %s\n", plchan->name);
 		plchan->state = PL08X_CHAN_WAITING;
+		plchan->waiting_at = jiffies;
 		return;
 	}
 
@@ -913,22 +916,29 @@ static void pl08x_phy_free(struct pl08x_dma_chan *plchan)
 {
 	struct pl08x_driver_data *pl08x = plchan->host;
 	struct pl08x_dma_chan *p, *next;
-
+	unsigned long waiting_at;
  retry:
 	next = NULL;
+	waiting_at = jiffies;
 
-	/* Find a waiting virtual channel for the next transfer. */
+	/*
+	 * Find a waiting virtual channel for the next transfer.
+	 * To be fair, time when each channel reached waiting state is compared
+	 * to select channel that is waiting for the longest time.
+	 */
 	list_for_each_entry(p, &pl08x->memcpy.channels, vc.chan.device_node)
-		if (p->state == PL08X_CHAN_WAITING) {
+		if (p->state == PL08X_CHAN_WAITING &&
+		    p->waiting_at <= waiting_at) {
 			next = p;
-			break;
+			waiting_at = p->waiting_at;
 		}
 
 	if (!next && pl08x->has_slave) {
 		list_for_each_entry(p, &pl08x->slave.channels, vc.chan.device_node)
-			if (p->state == PL08X_CHAN_WAITING) {
+			if (p->state == PL08X_CHAN_WAITING &&
+			    p->waiting_at <= waiting_at) {
 				next = p;
-				break;
+				waiting_at = p->waiting_at;
 			}
 	}