[PATCH] drivers/tty: amba-pl011: defer driver probing if external dma is not ready.

[Jorge Ramirez-Ortiz]

This patch addresses a race condition that happens when
device_initcall(pl011_dma_initicall) is executed before all the devices have been
probed - this issue was observed on an hisi_6220 SoC (HiKey board from Linaro).

The proposed implementation uses deferred driver probing to wait for the DMA
controller to be registered.

Signed-off-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz at linaro.org>
---
 drivers/tty/serial/amba-pl011.c | 70 +++++++++++------------------------------
 1 file changed, 18 insertions(+), 52 deletions(-)

diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
index 8d94c19..40219e5 100644
--- a/drivers/tty/serial/amba-pl011.c
+++ b/drivers/tty/serial/amba-pl011.c
@@ -29,6 +29,7 @@
  * and hooked into this driver.
  */
 
+#define pr_fmt(fmt) "amba-pl011: "fmt
 
 #if defined(CONFIG_SERIAL_AMBA_PL011_CONSOLE) && defined(CONFIG_MAGIC_SYSRQ)
 #define SUPPORT_SYSRQ
@@ -261,7 +262,7 @@ static void pl011_sgbuf_free(struct dma_chan *chan, struct pl011_sgbuf *sg,
 	}
 }
 
-static void pl011_dma_probe_initcall(struct device *dev, struct uart_amba_port *uap)
+static int pl011_dma_probe(struct device *dev, struct uart_amba_port *uap)
 {
 	/* DMA is the sole user of the platform data right now */
 	struct amba_pl011_data *plat = dev_get_platdata(uap->port.dev);
@@ -275,13 +276,17 @@ static void pl011_dma_probe_initcall(struct device *dev, struct uart_amba_port *
 	struct dma_chan *chan;
 	dma_cap_mask_t mask;
 
-	chan = dma_request_slave_channel(dev, "tx");
+	chan = dma_request_slave_channel_reason(dev, "tx");
+	if (IS_ERR(chan)) {
+
+		/* the dma driver has not been initialized yet */
+		if (PTR_ERR(chan) == -EPROBE_DEFER)
+			return -EPROBE_DEFER;
 
-	if (!chan) {
 		/* We need platform data */
 		if (!plat || !plat->dma_filter) {
 			dev_info(uap->port.dev, "no DMA platform data\n");
-			return;
+			return 0;
 		}
 
 		/* Try to acquire a generic DMA engine slave TX channel */
@@ -292,7 +297,7 @@ static void pl011_dma_probe_initcall(struct device *dev, struct uart_amba_port *
 						plat->dma_tx_param);
 		if (!chan) {
 			dev_err(uap->port.dev, "no TX DMA channel!\n");
-			return;
+			return 0;
 		}
 	}
 
@@ -310,7 +315,7 @@ static void pl011_dma_probe_initcall(struct device *dev, struct uart_amba_port *
 
 		if (!chan) {
 			dev_err(uap->port.dev, "no RX DMA channel!\n");
-			return;
+			return 0;
 		}
 	}
 
@@ -383,54 +388,10 @@ static void pl011_dma_probe_initcall(struct device *dev, struct uart_amba_port *
 		dev_info(uap->port.dev, "DMA channel RX %s\n",
 			 dma_chan_name(uap->dmarx.chan));
 	}
-}
-
-#ifndef MODULE
-/*
- * Stack up the UARTs and let the above initcall be done at device
- * initcall time, because the serial driver is called as an arch
- * initcall, and at this time the DMA subsystem is not yet registered.
- * At this point the driver will switch over to using DMA where desired.
- */
-struct dma_uap {
-	struct list_head node;
-	struct uart_amba_port *uap;
-	struct device *dev;
-};
-
-static LIST_HEAD(pl011_dma_uarts);
-
-static int __init pl011_dma_initcall(void)
-{
-	struct list_head *node, *tmp;
 
-	list_for_each_safe(node, tmp, &pl011_dma_uarts) {
-		struct dma_uap *dmau = list_entry(node, struct dma_uap, node);
-		pl011_dma_probe_initcall(dmau->dev, dmau->uap);
-		list_del(node);
-		kfree(dmau);
-	}
 	return 0;
 }
 
-device_initcall(pl011_dma_initcall);
-
-static void pl011_dma_probe(struct device *dev, struct uart_amba_port *uap)
-{
-	struct dma_uap *dmau = kzalloc(sizeof(struct dma_uap), GFP_KERNEL);
-	if (dmau) {
-		dmau->uap = uap;
-		dmau->dev = dev;
-		list_add_tail(&dmau->node, &pl011_dma_uarts);
-	}
-}
-#else
-static void pl011_dma_probe(struct device *dev, struct uart_amba_port *uap)
-{
-	pl011_dma_probe_initcall(dev, uap);
-}
-#endif
-
 static void pl011_dma_remove(struct uart_amba_port *uap)
 {
 	/* TODO: remove the initcall if it has not yet executed */
@@ -1142,8 +1103,9 @@ static inline bool pl011_dma_rx_running(struct uart_amba_port *uap)
 
 #else
 /* Blank functions if the DMA engine is not available */
-static inline void pl011_dma_probe(struct device *dev, struct uart_amba_port *uap)
+static int pl011_dma_probe(struct device *dev, struct uart_amba_port *uap)
 {
+	return 0;
 }
 
 static inline void pl011_dma_remove(struct uart_amba_port *uap)
@@ -2218,7 +2180,11 @@ static int pl011_probe(struct amba_device *dev, const struct amba_id *id)
 	uap->port.ops = &amba_pl011_pops;
 	uap->port.flags = UPF_BOOT_AUTOCONF;
 	uap->port.line = i;
-	pl011_dma_probe(&dev->dev, uap);
+	ret = pl011_dma_probe(&dev->dev, uap);
+	if (ret) {
+		devm_kfree(&dev->dev, uap);
+		return ret;
+	}
 
 	/* Ensure interrupts from this UART are masked and cleared */
 	writew(0, uap->port.membase + UART011_IMSC);
-- 
1.9.1