linux-kernel - [PATCH] tty: mips_ejtag_fdc: Call cpu_relax() in registers polling busy loops

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Message-ID: <20241219124254.321778-1-quic_zhonhan@quicinc.com>
Date: Thu, 19 Dec 2024 20:42:54 +0800
From: Zhongqiu Han <quic_zhonhan@...cinc.com>
To: <gregkh@...uxfoundation.org>, <jirislaby@...nel.org>
CC: <linux-kernel@...r.kernel.org>, <linux-serial@...r.kernel.org>,
        <quic_zhonhan@...cinc.com>
Subject: [PATCH] tty: mips_ejtag_fdc: Call cpu_relax() in registers polling busy loops

It is considered good practice to call cpu_relax() in busy loops, see
Documentation/process/volatile-considered-harmful.rst. This can lower CPU
power consumption or yield to a hyperthreaded twin processor, or serve as
a compiler barrier. In addition, if something goes wrong in the busy loop
at least it can prevent things from getting worse.

Signed-off-by: Zhongqiu Han <quic_zhonhan@...cinc.com>
---
 drivers/tty/mips_ejtag_fdc.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/mips_ejtag_fdc.c b/drivers/tty/mips_ejtag_fdc.c
index afbf7738c7c4..b17ead1e9698 100644
--- a/drivers/tty/mips_ejtag_fdc.c
+++ b/drivers/tty/mips_ejtag_fdc.c
@@ -346,7 +346,7 @@ static void mips_ejtag_fdc_console_write(struct console *c, const char *s,
 
 		/* Busy wait until there's space in fifo */
 		while (__raw_readl(regs + REG_FDSTAT) & REG_FDSTAT_TXF)
-			;
+			cpu_relax();
 		__raw_writel(word.word, regs + REG_FDTX(c->index));
 	}
 out:
@@ -1233,7 +1233,7 @@ static void kgdbfdc_push_one(void)
 
 	/* Busy wait until there's space in fifo */
 	while (__raw_readl(regs + REG_FDSTAT) & REG_FDSTAT_TXF)
-		;
+		cpu_relax();
 	__raw_writel(word.word,
 		     regs + REG_FDTX(CONFIG_MIPS_EJTAG_FDC_KGDB_CHAN));
 }
-- 
2.25.1