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From:   Josua Mayer <josua@solid-run.com>
To:     linux-wireless@vger.kernel.org
Cc:     p.zabel@pengutronix.de, Josua Mayer <josua@solid-run.com>,
        Johannes Berg <johannes@sipsolutions.net>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        "John W. Linville" <linville@tuxdriver.com>,
        Rhyland Klein <rklein@nvidia.com>
Subject: [PATCH] net: rfkill: gpio: prevent value glitch during probe
Date:   Wed,  4 Oct 2023 18:39:28 +0200
Message-Id: <20231004163928.14609-1-josua@solid-run.com>
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When either reset- or shutdown-gpio have are initially deasserted,
e.g. after a reboot - or when the hardware does not include pull-down,
there will be a short toggle of both IOs to logical 0 and back to 1.

It seems that the rfkill default is unblocked, so the driver should not
glitch to output low during probe.
It can lead e.g. to unexpected lte modem reconnect:

[1] root@localhost:~# dmesg | grep "usb 2-1"
[    2.136124] usb 2-1: new SuperSpeed USB device number 2 using xhci-hcd
[   21.215278] usb 2-1: USB disconnect, device number 2
[   28.833977] usb 2-1: new SuperSpeed USB device number 3 using xhci-hcd

The glitch has been discovered on an arm64 board, now that device-tree
support for the rfkill-gpio driver has finally appeared :).

Change the flags for devm_gpiod_get_optional from GPIOD_OUT_LOW to
GPIOD_ASIS to avoid any glitches.
The rfkill driver will set the intended value during rfkill_sync_work.

Fixes: 7176ba23f8b5 ("net: rfkill: add generic gpio rfkill driver")
Signed-off-by: Josua Mayer <josua@solid-run.com>
---
 net/rfkill/rfkill-gpio.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/rfkill/rfkill-gpio.c b/net/rfkill/rfkill-gpio.c
index e9d1b2f2ff0a..5a81505fba9a 100644
--- a/net/rfkill/rfkill-gpio.c
+++ b/net/rfkill/rfkill-gpio.c
@@ -108,13 +108,13 @@ static int rfkill_gpio_probe(struct platform_device *pdev)
 
 	rfkill->clk = devm_clk_get(&pdev->dev, NULL);
 
-	gpio = devm_gpiod_get_optional(&pdev->dev, "reset", GPIOD_OUT_LOW);
+	gpio = devm_gpiod_get_optional(&pdev->dev, "reset", GPIOD_ASIS);
 	if (IS_ERR(gpio))
 		return PTR_ERR(gpio);
 
 	rfkill->reset_gpio = gpio;
 
-	gpio = devm_gpiod_get_optional(&pdev->dev, "shutdown", GPIOD_OUT_LOW);
+	gpio = devm_gpiod_get_optional(&pdev->dev, "shutdown", GPIOD_ASIS);
 	if (IS_ERR(gpio))
 		return PTR_ERR(gpio);
 
-- 
2.35.3