linux-kernel - [PATCH 1/2] usb: dwc3: core: Call cpu_relax() in registers polling busy loops

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Message-ID: <20240820121501.3593245-2-quic_zhonhan@quicinc.com>
Date: Tue, 20 Aug 2024 20:15:00 +0800
From: Zhongqiu Han <quic_zhonhan@...cinc.com>
To: <Thinh.Nguyen@...opsys.com>, <gregkh@...uxfoundation.org>,
        <mathias.nyman@...el.com>
CC: <linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <quic_zhonhan@...cinc.com>
Subject: [PATCH 1/2] usb: dwc3: core: Call cpu_relax() in registers polling busy loops

Busy loops that poll on a register should call cpu_relax(). On some
architectures, it can lower CPU power consumption or yield to a
hyperthreaded twin processor. It also serves as a compiler barrier,
see Documentation/process/volatile-considered-harmful.rst. 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/usb/dwc3/core.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/usb/dwc3/core.c b/drivers/usb/dwc3/core.c
index 734de2a8bd21..498f08dbbdb5 100644
--- a/drivers/usb/dwc3/core.c
+++ b/drivers/usb/dwc3/core.c
@@ -2050,6 +2050,8 @@ static int dwc3_get_num_ports(struct dwc3 *dwc)
 		if (!offset)
 			break;
 
+		cpu_relax();
+
 		val = readl(base + offset);
 		major_revision = XHCI_EXT_PORT_MAJOR(val);
 
-- 
2.25.1