diff options
Diffstat (limited to 'target/linux/bcm27xx/patches-5.4/950-0424-tty-amba-pl011-Avoid-rare-write-when-full-error.patch')
| -rw-r--r-- | target/linux/bcm27xx/patches-5.4/950-0424-tty-amba-pl011-Avoid-rare-write-when-full-error.patch | 42 |
1 files changed, 42 insertions, 0 deletions
diff --git a/target/linux/bcm27xx/patches-5.4/950-0424-tty-amba-pl011-Avoid-rare-write-when-full-error.patch b/target/linux/bcm27xx/patches-5.4/950-0424-tty-amba-pl011-Avoid-rare-write-when-full-error.patch new file mode 100644 index 0000000000..b2b27f258f --- /dev/null +++ b/target/linux/bcm27xx/patches-5.4/950-0424-tty-amba-pl011-Avoid-rare-write-when-full-error.patch @@ -0,0 +1,42 @@ +From 66ca4b2544dbd4f10d8f387782f5c7200d1e2167 Mon Sep 17 00:00:00 2001 +From: Phil Elwell <phil@raspberrypi.org> +Date: Wed, 29 Jan 2020 09:35:19 +0000 +Subject: [PATCH] tty: amba-pl011: Avoid rare write-when-full error + +Under some circumstances on BCM283x processors data loss can be +observed - a single byte missing from the TX output stream. These bytes +are always the last byte of a batch of 8 written from pl011_tx_chars +when from_irq is true, meaning that the FIFO full flag is not checked +before writing. + +The transmit optimisation relies on the FIFO being half-empty when the +TX interrupt is raised. Instrumenting the driver further showed that +the failure case correlated with the TX FIFO full flag being set at the +point where the last byte was written to the data register, which +explains the data loss but not how the FIFO appeared to be prematurely +full. A possible explanation is that a FIFO write was in flight at the +time the interrupt was raised, but as yet there is no hypothesis as to +how this might occur. + +In the absence of a clear understanding of the failure mechanism, avoid +the problem by checking the FIFO levels before writing the last byte of +the group, which will have minimal performance impact. + +Signed-off-by: Phil Elwell <phil@raspberrypi.org> +--- + drivers/tty/serial/amba-pl011.c | 4 ++++ + 1 file changed, 4 insertions(+) + +--- a/drivers/tty/serial/amba-pl011.c ++++ b/drivers/tty/serial/amba-pl011.c +@@ -1444,6 +1444,10 @@ static bool pl011_tx_chars(struct uart_a + if (likely(from_irq) && count-- == 0) + break; + ++ if (likely(from_irq) && count == 0 && ++ pl011_read(uap, REG_FR) & UART01x_FR_TXFF) ++ break; ++ + if (!pl011_tx_char(uap, xmit->buf[xmit->tail], from_irq)) + break; + |