Currently, the pl011 earlycon implementation waits for the UART transmitter to drain completely and become idle after each character is written. This can result in (mostly harmless) delays and stuttering output on the wire, and can also lead to poor performance in virtualised UART implementations: thrashing between VMs can occur as each character gets forcibly drained to the remove sink (dom0 in the Xen case) before the source guest writes another character. However, the semantics of earlycon don't allow callers to write one character at a time with the expectation of completion: the only interface to exposed to earlycon writes a whole string before returning. So, this patch eliminates the draining of the transmitted from pl011_putc() and moves if to the the and of pl011_early_write() instead. From the earlycon caller's point of view, the effect should be faster but otherwise identical: either way, all the characters have gone out onto the wire before the write method returns. Signed-off-by: Dave Martin <Dave.Martin@xxxxxxx> --- For background on the virtualisation issue, see [1]. Although virtual UART implementations should be working around this issue somehow for compatibility with current and older kernels, this patch should promote better interoperation, and earlycon throughput should improve a bit on real hardware too. [1] Re: [Xen-devel] [PATCH 26/27 v12] arm/xen: vpl011: Fix the slow early console SBSA UART output https://lists.xenproject.org/archives/html/xen-devel/2017-10/msg01949.html --- drivers/tty/serial/amba-pl011.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c index 084ed3f..e27059a 100644 --- a/drivers/tty/serial/amba-pl011.c +++ b/drivers/tty/serial/amba-pl011.c @@ -2444,16 +2444,18 @@ static void pl011_putc(struct uart_port *port, int c) while (__pl011_read(port, reg_offset, REG_FR) & UART01x_FR_TXFF) cpu_relax(); __pl011_write(c, port, reg_offset, REG_DR); - while (!__pl011_tx_empty(port, reg_offset, vendor)) - cpu_relax(); } static void pl011_early_write(struct console *con, const char *s, unsigned n) { struct earlycon_device *dev = con->data; + const struct vendor_data *vendor = dev->port.private_data; + const u16 *reg_offset = vendor->reg_offset; wmb(); uart_console_write(&dev->port, s, n, pl011_putc); + while (!__pl011_tx_empty(&dev->port, reg_offset, vendor)) + cpu_relax(); } static int __init -- 2.1.4 -- To unsubscribe from this list: send the line "unsubscribe linux-serial" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
