dmaengine: rcar-dmac: Handle hardware descriptor allocation failure

If the atomic DMA coherent pool is too small, disable use of hardware
descriptor lists instead of crashing the system:

ERROR: 256 KiB atomic DMA coherent pool is too small!
Please increase it with coherent_pool= kernel parameter!

Unable to handle kernel NULL pointer dereference at virtual address 00000004
Internal error: Oops: a07 [#1] PREEMPT SMP ARM

PC is at rcar_dmac_chan_reinit+0x3c/0x160
LR is at _raw_spin_lock_irqsave+0x18/0x5c

[<802132c0>] (rcar_dmac_chan_reinit) from [<80214818>] (rcar_dmac_isr_error+0x84/0xa0)
[<80214818>] (rcar_dmac_isr_error) from [<80060484>] (handle_irq_event_percpu+0x50/0x150)
[<80060484>] (handle_irq_event_percpu) from [<800605c0>] (handle_irq_event+0x3c/0x5c)
[<800605c0>] (handle_irq_event) from [<8006350c>] (handle_fasteoi_irq+0xb8/0x198)
[<8006350c>] (handle_fasteoi_irq) from [<8005fdb0>] (generic_handle_irq+0x20/0x30)
[<8005fdb0>] (generic_handle_irq) from [<8000fcd0>] (handle_IRQ+0x50/0xc4)
[<8000fcd0>] (handle_IRQ) from [<800092cc>] (gic_handle_irq+0x28/0x5c)
[<800092cc>] (gic_handle_irq) from [<80012700>] (__irq_svc+0x40/0x70)

Kernel panic - not syncing: Fatal exception in interrupt
Signed-off-by: Jürg Billeter <j@bitron.ch>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>

drivers/dma/sh/rcar-dmac.c

@@ -679,8 +679,8 @@ static void rcar_dmac_realloc_hwdesc(struct rcar_dmac_chan *chan,
 	desc->hwdescs.size = size;
 }
 
-static void rcar_dmac_fill_hwdesc(struct rcar_dmac_chan *chan,
-				  struct rcar_dmac_desc *desc)
+static int rcar_dmac_fill_hwdesc(struct rcar_dmac_chan *chan,
+				 struct rcar_dmac_desc *desc)
 {
 	struct rcar_dmac_xfer_chunk *chunk;
 	struct rcar_dmac_hw_desc *hwdesc;
@@ -689,7 +689,7 @@ static void rcar_dmac_fill_hwdesc(struct rcar_dmac_chan *chan,
 
 	hwdesc = desc->hwdescs.mem;
 	if (!hwdesc)
-		return;
+		return -ENOMEM;
 
 	list_for_each_entry(chunk, &desc->chunks, node) {
 		hwdesc->sar = chunk->src_addr;
@@ -697,6 +697,8 @@ static void rcar_dmac_fill_hwdesc(struct rcar_dmac_chan *chan,
 		hwdesc->tcr = chunk->size >> desc->xfer_shift;
 		hwdesc++;
 	}
+
+	return 0;
 }
 
 /* -----------------------------------------------------------------------------
@@ -917,8 +919,10 @@ rcar_dmac_chan_prep_sg(struct rcar_dmac_chan *chan, struct scatterlist *sgl,
 	 * additional complexity remains to be investigated.
 	 */
 	desc->hwdescs.use = !highmem && nchunks > 1;
-	if (desc->hwdescs.use)
-		rcar_dmac_fill_hwdesc(chan, desc);
+	if (desc->hwdescs.use) {
+		if (rcar_dmac_fill_hwdesc(chan, desc) < 0)
+			desc->hwdescs.use = false;
+	}
 
 	return &desc->async_tx;
 }