--- /dev/null
+From 3e3c13488e4efa0236c47a98ee5e759bf1f7c757 Mon Sep 17 00:00:00 2001
+From: Phil Elwell <phil@raspberrypi.org>
+Date: Thu, 11 Jul 2019 13:13:39 +0100
+Subject: [PATCH] tty: amba-pl011: Make TX optimisation conditional
+
+pl011_tx_chars takes a "from_irq" parameter to reduce the number of
+register accesses. When from_irq is true the function assumes that the
+FIFO is half empty and writes up to half a FIFO's worth of bytes
+without polling the FIFO status register, the reasoning being that
+the function is being called as a result of the TX interrupt being
+raised. This logic would work were it not for the fact that
+pl011_rx_chars, called from pl011_int before pl011_tx_chars, releases
+the spinlock before calling tty_flip_buffer_push.
+
+A user thread writing to the UART claims the spinlock and ultimately
+calls pl011_tx_chars with from_irq set to false. This reverts to the
+older logic that polls the FIFO status register before sending every
+byte. If this happen on an SMP system during the section of the IRQ
+handler where the spinlock has been released, then by the time the TX
+interrupt handler is called, the FIFO may already be full, and any
+further writes are likely to be lost.
+
+The fix involves adding a per-port flag that is true iff running from
+within the interrupt handler and the spinlock has not yet been released.
+This flag is then used as the value for the from_irq parameter of
+pl011_tx_chars, causing polling to be used in the unsafe case.
+
+Fixes: 1e84d22322ce ("serial/amba-pl011: Refactor and simplify TX FIFO handling")
+
+Signed-off-by: Phil Elwell <phil@raspberrypi.org>
+---
+ drivers/tty/serial/amba-pl011.c | 7 ++++++-
+ 1 file changed, 6 insertions(+), 1 deletion(-)
+
+--- a/drivers/tty/serial/amba-pl011.c
++++ b/drivers/tty/serial/amba-pl011.c
+@@ -270,6 +270,7 @@ struct uart_amba_port {
+ unsigned int old_cr; /* state during shutdown */
+ unsigned int fixed_baud; /* vendor-set fixed baud rate */
+ char type[12];
++ bool irq_locked; /* in irq, unreleased lock */
+ #ifdef CONFIG_DMA_ENGINE
+ /* DMA stuff */
+ bool using_tx_dma;
+@@ -813,6 +814,7 @@ __acquires(&uap->port.lock)
+ if (!uap->using_tx_dma)
+ return;
+
++ uap->irq_locked = 0;
+ dmaengine_terminate_async(uap->dmatx.chan);
+
+ if (uap->dmatx.queued) {
+@@ -939,6 +941,7 @@ static void pl011_dma_rx_chars(struct ua
+ fifotaken = pl011_fifo_to_tty(uap);
+ }
+
++ uap->irq_locked = 0;
+ spin_unlock(&uap->port.lock);
+ dev_vdbg(uap->port.dev,
+ "Took %d chars from DMA buffer and %d chars from the FIFO\n",
+@@ -1347,6 +1350,7 @@ __acquires(&uap->port.lock)
+ {
+ pl011_fifo_to_tty(uap);
+
++ uap->irq_locked = 0;
+ spin_unlock(&uap->port.lock);
+ tty_flip_buffer_push(&uap->port.state->port);
+ /*
+@@ -1482,6 +1486,7 @@ static irqreturn_t pl011_int(int irq, vo
+ int handled = 0;
+
+ spin_lock_irqsave(&uap->port.lock, flags);
++ uap->irq_locked = 1;
+ status = pl011_read(uap, REG_RIS) & uap->im;
+ if (status) {
+ do {
+@@ -1501,7 +1506,7 @@ static irqreturn_t pl011_int(int irq, vo
+ UART011_CTSMIS|UART011_RIMIS))
+ pl011_modem_status(uap);
+ if (status & UART011_TXIS)
+- pl011_tx_chars(uap, true);
++ pl011_tx_chars(uap, uap->irq_locked);
+
+ if (pass_counter-- == 0)
+ break;
projects / openwrt / staging / chunkeey.git / blobdiff