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86 lines
3.1 KiB
Diff
86 lines
3.1 KiB
Diff
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From 76e24a2218069fadb28c0c2f5d813302ad5f85a3 Mon Sep 17 00:00:00 2001
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From: Phil Elwell <phil@raspberrypi.org>
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Date: Thu, 11 Jul 2019 13:13:39 +0100
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Subject: [PATCH] tty: amba-pl011: Make TX optimisation conditional
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pl011_tx_chars takes a "from_irq" parameter to reduce the number of
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register accesses. When from_irq is true the function assumes that the
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FIFO is half empty and writes up to half a FIFO's worth of bytes
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without polling the FIFO status register, the reasoning being that
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the function is being called as a result of the TX interrupt being
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raised. This logic would work were it not for the fact that
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pl011_rx_chars, called from pl011_int before pl011_tx_chars, releases
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the spinlock before calling tty_flip_buffer_push.
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A user thread writing to the UART claims the spinlock and ultimately
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calls pl011_tx_chars with from_irq set to false. This reverts to the
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older logic that polls the FIFO status register before sending every
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byte. If this happen on an SMP system during the section of the IRQ
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handler where the spinlock has been released, then by the time the TX
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interrupt handler is called, the FIFO may already be full, and any
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further writes are likely to be lost.
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The fix involves adding a per-port flag that is true iff running from
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within the interrupt handler and the spinlock has not yet been released.
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This flag is then used as the value for the from_irq parameter of
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pl011_tx_chars, causing polling to be used in the unsafe case.
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Fixes: 1e84d22322ce ("serial/amba-pl011: Refactor and simplify TX FIFO handling")
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Signed-off-by: Phil Elwell <phil@raspberrypi.org>
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---
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drivers/tty/serial/amba-pl011.c | 7 ++++++-
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1 file changed, 6 insertions(+), 1 deletion(-)
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--- a/drivers/tty/serial/amba-pl011.c
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+++ b/drivers/tty/serial/amba-pl011.c
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@@ -270,6 +270,7 @@ struct uart_amba_port {
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unsigned int old_cr; /* state during shutdown */
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unsigned int fixed_baud; /* vendor-set fixed baud rate */
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char type[12];
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+ bool irq_locked; /* in irq, unreleased lock */
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#ifdef CONFIG_DMA_ENGINE
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/* DMA stuff */
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bool using_tx_dma;
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@@ -813,6 +814,7 @@ __acquires(&uap->port.lock)
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if (!uap->using_tx_dma)
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return;
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+ uap->irq_locked = 0;
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dmaengine_terminate_async(uap->dmatx.chan);
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if (uap->dmatx.queued) {
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@@ -939,6 +941,7 @@ static void pl011_dma_rx_chars(struct ua
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fifotaken = pl011_fifo_to_tty(uap);
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}
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+ uap->irq_locked = 0;
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spin_unlock(&uap->port.lock);
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dev_vdbg(uap->port.dev,
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"Took %d chars from DMA buffer and %d chars from the FIFO\n",
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@@ -1347,6 +1350,7 @@ __acquires(&uap->port.lock)
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{
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pl011_fifo_to_tty(uap);
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+ uap->irq_locked = 0;
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spin_unlock(&uap->port.lock);
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tty_flip_buffer_push(&uap->port.state->port);
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/*
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@@ -1482,6 +1486,7 @@ static irqreturn_t pl011_int(int irq, vo
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int handled = 0;
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spin_lock_irqsave(&uap->port.lock, flags);
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+ uap->irq_locked = 1;
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status = pl011_read(uap, REG_RIS) & uap->im;
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if (status) {
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do {
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@@ -1501,7 +1506,7 @@ static irqreturn_t pl011_int(int irq, vo
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UART011_CTSMIS|UART011_RIMIS))
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pl011_modem_status(uap);
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if (status & UART011_TXIS)
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- pl011_tx_chars(uap, true);
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+ pl011_tx_chars(uap, uap->irq_locked);
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if (pass_counter-- == 0)
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break;
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