netdev - Re: [PATCH] net: dsa: lan9303: fix reset on probe

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Message-ID: <yw1x1r0c5794.fsf@mansr.com>
Date:   Wed, 09 Feb 2022 16:34:15 +0000
From:   Måns Rullgård <mans@...sr.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Vladimir Oltean <olteanv@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Juergen Borleis <kernel@...gutronix.de>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: dsa: lan9303: fix reset on probe

Andrew Lunn <andrew@...n.ch> writes:

> On Wed, Feb 09, 2022 at 02:54:54PM +0000, Mans Rullgard wrote:
>> The reset input to the LAN9303 chip is active low, and devicetree
>> gpio handles reflect this.  Therefore, the gpio should be requested
>> with an initial state of high in order for the reset signal to be
>> asserted.  Other uses of the gpio already use the correct polarity.
>> 
>> Signed-off-by: Mans Rullgard <mans@...sr.com>
>> ---
>>  drivers/net/dsa/lan9303-core.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/drivers/net/dsa/lan9303-core.c b/drivers/net/dsa/lan9303-core.c
>> index aa1142d6a9f5..2de67708bbd2 100644
>> --- a/drivers/net/dsa/lan9303-core.c
>> +++ b/drivers/net/dsa/lan9303-core.c
>> @@ -1301,7 +1301,7 @@ static int lan9303_probe_reset_gpio(struct lan9303 *chip,
>>  				     struct device_node *np)
>>  {
>>  	chip->reset_gpio = devm_gpiod_get_optional(chip->dev, "reset",
>> -						   GPIOD_OUT_LOW);
>> +						   GPIOD_OUT_HIGH);
>>  	if (IS_ERR(chip->reset_gpio))
>>  		return PTR_ERR(chip->reset_gpio);
>
> lan9303_handle_reset() does a sleep and then releases the reset. I
> don't see anywhere in the driver which asserts the reset first. So is
> it actually asserted as part of this getting the GPIO? And if so, does
> not this change actually break the reset?

The GPIOD_OUT_xxx flags to gpiod_get() request that the pin be
configured as output and set to high/low initially.  The GPIOD_OUT_LOW
currently used by the lan9303 driver together with GPIO_ACTIVE_LOW in
the devicetrees results in the actual voltage being set high.  The
driver then sleeps for a bit before setting the gpio value to zero,
again translated to a high output voltage.  That is, the value set after
the sleep is the same as it was initially.  This is obviously not the
intent.

With the patch applied, I can measure the reset signal pulse low for the
configured duration when the device is probed.  Without the patch, the
reset signal remains high and no reset of the device occurs.

-- 
Måns Rullgård