linux-kernel - Re: [PATCH v2 4/6] ARM: at91/dt: sama5d3: add i2c gpio pinctrl

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Date:   Mon, 6 Jan 2020 16:58:39 +0000
From:   <Codrin.Ciubotariu@...rochip.com>
To:     <peda@...ntia.se>, <linux-i2c@...r.kernel.org>,
        <devicetree@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>
CC:     <kamel.bouhara@...tlin.com>, <wsa@...-dreams.de>,
        <Nicolas.Ferre@...rochip.com>, <alexandre.belloni@...tlin.com>,
        <Ludovic.Desroches@...rochip.com>, <robh@...nel.org>
Subject: Re: [PATCH v2 4/6] ARM: at91/dt: sama5d3: add i2c gpio pinctrl

On 05.01.2020 00:39, Peter Rosin wrote:
> On 2020-01-03 10:49, Codrin.Ciubotariu@...rochip.com wrote:
>> From: Kamel Bouhara <kamel.bouhara@...tlin.com>
>>
>> Add the i2c gpio pinctrls to support the i2c bus recovery
>>
>> Signed-off-by: Kamel Bouhara <kamel.bouhara@...tlin.com>
>> ---
>>
>> Changes in v2:
>>   - none;
>>
>>   arch/arm/boot/dts/sama5d3.dtsi | 33 ++++++++++++++++++++++++++++++---
>>   1 file changed, 30 insertions(+), 3 deletions(-)
>>
> 
> *snip*
> 
>> @@ -639,6 +648,12 @@
>>                                                        <AT91_PIOA 30 AT91_PERIPH_A AT91_PINCTRL_NONE   /* PA30 periph A TWD0 pin, conflicts with URXD1, ISI_VSYNC */
>>                                                         AT91_PIOA 31 AT91_PERIPH_A AT91_PINCTRL_NONE>; /* PA31 periph A TWCK0 pin, conflicts with UTXD1, ISI_HSYNC */
>>                                        };
>> +
>> +                                     pinctrl_i2c0_gpio: i2c0-gpio {
>> +                                             atmel,pins =
>> +                                                     <AT91_PIOA 30 AT91_PERIPH_GPIO AT91_PINCTRL_PULL_UP
>> +                                                      AT91_PIOA 31 AT91_PERIPH_GPIO AT91_PINCTRL_PULL_UP>;
>> +                                     };
> 
> I'm curious, but why are pull-ups suddenly needed just because the pins are
> used for GPIO recovery? Why are pull-ups not needed when the pins are used
> by the I2C peripheral device(s)?
> 
> Given figure 27-2 "I/O Line Control Logic" in my SAMA5D3 datasheet, I see
> no difference as to how and why the pull-ups are applied depending on what
> the current function of the pin is. So, if the I2C bus works w/o pulls, bus
> recovery using GPIO must also work w/o pulls.
> 
> I.e. the device tree requires you to have external pull-ups on the I2C bus
> anyway, so why bother with internal pull-ups for the recovery case?
> 
> Changing pull-up settings just for recovery feels like something that will
> inevitably create hard to debug surprises at the least opportune time...
> 
> Or am I missing something?
> 
> (I'm focusing on SAMA5D3 since that is what I happen to work with,
>   but the same question appears to apply for SAMA5D2 and SAMA5D4...)

I don't think we need the pull-ups. I will remove them in v3.

Thanks and best regards,
Codrin