dmaengine: cppi41: Fix oops in cppi41_runtime_resume

Commit fdea2d09 ("dmaengine: cppi41: Add basic PM runtime support")
together with recent MUSB changes allowed USB and DMA on BeagleBone to idle
when no cable is connected. But looks like few corner case issues still
remain.

Looks like just by re-plugging USB cable about ten or so times on BeagleBone
when configured in USB peripheral mode we can get warnings and eventually
trigger an oops in cppi41 DMA:

WARNING: CPU: 0 PID: 14 at drivers/dma/cppi41.c:1154 cppi41_runtime_suspend+
x28/0x38 [cppi41]
...

WARNING: CPU: 0 PID: 14 at drivers/dma/cppi41.c:452
push_desc_queue+0x94/0x9c [cppi41]
...

Unable to handle kernel NULL pointer dereference at virtual
address 00000104
pgd = c0004000
[00000104] *pgd=00000000
Internal error: Oops: 805 [#1] SMP ARM
...
[<bf0d92cc>] (cppi41_runtime_resume [cppi41]) from [<c0589838>]
(__rpm_callback+0xc0/0x214)
[<c0589838>] (__rpm_callback) from [<c05899ac>] (rpm_callback+0x20/0x80)
[<c05899ac>] (rpm_callback) from [<c0589460>] (rpm_resume+0x504/0x78c)
[<c0589460>] (rpm_resume) from [<c058a1a0>] (pm_runtime_work+0x60/0xa8)
[<c058a1a0>] (pm_runtime_work) from [<c0156120>] (process_one_work+0x2b4/0x808)

This is because of a race with runtime PM and cppi41_dma_issue_pending()
as reported by Alexandre Bailon <abailon@baylibre.com> in earlier
set of patches. Based on mailing list discussions we however came to the
conclusion that a different fix from Alexandre's fix is needed in order
to guarantee that DMA is really active when we try to use it.

To fix the issue, we need to add a driver specific flag as we otherwise
can have -EINPROGRESS state set by runtime PM and can't rely on
pm_runtime_active() to tell us when we can use the DMA.

And we need to make sure the DMA transfers get triggered in the queued
order. So let's always queue the transfers, then flush the queue
from both cppi41_dma_issue_pending() and cppi41_runtime_resume()
as suggested by Grygorii Strashko <grygorii.strashko@ti.com> in an
earlier example patch.

For reference, this is also documented in Documentation/power/runtime_pm.txt
in the example at the end of the file as pointed out by Grygorii Strashko
<grygorii.strashko@ti.com>.

Based on earlier patches from Alexandre Bailon <abailon@baylibre.com>
and Grygorii Strashko <grygorii.strashko@ti.com> modified based on
testing and what was discussed on the mailing lists.

Fixes: fdea2d09 ("dmaengine: cppi41: Add basic PM runtime support")
Cc: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Bin Liu <b-liu@ti.com>
Cc: Grygorii Strashko <grygorii.strashko@ti.com>
Cc: Kevin Hilman <khilman@baylibre.com>
Cc: Patrick Titiano <ptitiano@baylibre.com>
Cc: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reported-by: Alexandre Bailon <abailon@baylibre.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Tested-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>

drivers/dma/cppi41.c

@@ -153,6 +153,8 @@ struct cppi41_dd {
 
 	/* context for suspend/resume */
 	unsigned int dma_tdfdq;
+
+	bool is_suspended;
 };
 
 #define FIST_COMPLETION_QUEUE	93
@@ -470,20 +472,26 @@ static void push_desc_queue(struct cppi41_channel *c)
 	cppi_writel(reg, cdd->qmgr_mem + QMGR_QUEUE_D(c->q_num));
 }
 
-static void pending_desc(struct cppi41_channel *c)
+/*
+ * Caller must hold cdd->lock to prevent push_desc_queue()
+ * getting called out of order. We have both cppi41_dma_issue_pending()
+ * and cppi41_runtime_resume() call this function.
+ */
+static void cppi41_run_queue(struct cppi41_dd *cdd)
 {
-	struct cppi41_dd *cdd = c->cdd;
-	unsigned long flags;
+	struct cppi41_channel *c, *_c;
 
-	spin_lock_irqsave(&cdd->lock, flags);
-	list_add_tail(&c->node, &cdd->pending);
-	spin_unlock_irqrestore(&cdd->lock, flags);
+	list_for_each_entry_safe(c, _c, &cdd->pending, node) {
+		push_desc_queue(c);
+		list_del(&c->node);
+	}
 }
 
 static void cppi41_dma_issue_pending(struct dma_chan *chan)
 {
 	struct cppi41_channel *c = to_cpp41_chan(chan);
 	struct cppi41_dd *cdd = c->cdd;
+	unsigned long flags;
 	int error;
 
 	error = pm_runtime_get(cdd->ddev.dev);
@@ -495,10 +503,11 @@ static void cppi41_dma_issue_pending(struct dma_chan *chan)
 		return;
 	}
 
-	if (likely(pm_runtime_active(cdd->ddev.dev)))
-		push_desc_queue(c);
-	else
-		pending_desc(c);
+	spin_lock_irqsave(&cdd->lock, flags);
+	list_add_tail(&c->node, &cdd->pending);
+	if (!cdd->is_suspended)
+		cppi41_run_queue(cdd);
+	spin_unlock_irqrestore(&cdd->lock, flags);
 
 	pm_runtime_mark_last_busy(cdd->ddev.dev);
 	pm_runtime_put_autosuspend(cdd->ddev.dev);
@@ -1166,8 +1175,12 @@ static int __maybe_unused cppi41_resume(struct device *dev)
 static int __maybe_unused cppi41_runtime_suspend(struct device *dev)
 {
 	struct cppi41_dd *cdd = dev_get_drvdata(dev);
+	unsigned long flags;
 
+	spin_lock_irqsave(&cdd->lock, flags);
+	cdd->is_suspended = true;
 	WARN_ON(!list_empty(&cdd->pending));
+	spin_unlock_irqrestore(&cdd->lock, flags);
 
 	return 0;
 }
@@ -1175,14 +1188,11 @@ static int __maybe_unused cppi41_runtime_suspend(struct device *dev)
 static int __maybe_unused cppi41_runtime_resume(struct device *dev)
 {
 	struct cppi41_dd *cdd = dev_get_drvdata(dev);
-	struct cppi41_channel *c, *_c;
 	unsigned long flags;
 
 	spin_lock_irqsave(&cdd->lock, flags);
-	list_for_each_entry_safe(c, _c, &cdd->pending, node) {
-		push_desc_queue(c);
-		list_del(&c->node);
-	}
+	cdd->is_suspended = false;
+	cppi41_run_queue(cdd);
 	spin_unlock_irqrestore(&cdd->lock, flags);
 
 	return 0;