linux-kernel - Re: [PATCH 3/4] usb: typec: add TUSB320xA driver

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Message-ID: <87lexlcsrj.fsf@bang-olufsen.dk>
Date:   Mon, 7 Mar 2022 22:17:04 +0000
From:   Alvin Šipraga <ALSI@...g-olufsen.dk>
To:     Heikki Krogerus <heikki.krogerus@...ux.intel.com>
CC:     Alvin Šipraga <alvin@...s.dk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rob Herring <robh+dt@...nel.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/4] usb: typec: add TUSB320xA driver

Hi Heikki,

Heikki Krogerus <heikki.krogerus@...ux.intel.com> writes:

> Hi,
>
> On Tue, Mar 01, 2022 at 02:20:07PM +0100, Alvin Šipraga wrote:
>> From: Alvin Šipraga <alsi@...g-olufsen.dk>
>> 
>> The TUSB320LA and TUSB320HA (or LAI, HAI) chips are I2C controlled
>> non-PD Type-C port controllers. They support detection of cable
>> orientation, port attachment state, and role, including Audio Accessory
>> and Debug Accessory modes. Add a typec class driver for this family.
>> 
>> Note that there already exists an extcon driver for the TUSB320 (a
>> slightly older revision that does not support setting role preference or
>> disabling the CC state machine). This driver is loosely based on that
>> one.
>
> This looked mostly OK to me. There is one question below.
>
> <snip>
>
>> +static int tusb320xa_check_signature(struct tusb320xa *tusb)
>> +{
>> +	static const char sig[] = { '\0', 'T', 'U', 'S', 'B', '3', '2', '0' };
>> +	unsigned int val;
>> +	int i, ret;
>> +
>> +	mutex_lock(&tusb->lock);
>> +
>> +	for (i = 0; i < sizeof(sig); i++) {
>> +		ret = regmap_read(tusb->regmap, sizeof(sig) - 1 - i, &val);
>> +		if (ret)
>> +			goto done;
>> +
>> +		if (val != sig[i]) {
>> +			dev_err(tusb->dev, "signature mismatch!\n");
>> +			ret = -ENODEV;
>> +			goto done;
>> +		}
>> +	}
>> +
>> +done:
>> +	mutex_unlock(&tusb->lock);
>> +
>> +	return ret;
>> +}
>
> Couldn't that be done with a single read?
>
>         char sig[8];
>         u64 val;
>
>         strcpy(sig, "TUSB320")
>
>         mutex_lock(&tusb->lock);
>
>         ret = regmap_raw_read(tusb->regmap, 0, &val, sizeof(val));
>         ...
>         if (val != cpu_to_le64(*(u64 *)sig)) {
>         ...
>
> Something like that?

I think it's a bit cryptic - are you sure it's worth it just to save 8
one-off regmap_read()s? I could also just remove this check... I see it
mostly as a courtesy to the user in case the I2C address in his device
tree mistakenly points to some other unsuspecting chip.

BTW, do you have any feedback on the device tree bindings of this
series? Rob had some questions and I am not sure that my proposed
bindings are fully aligned with the typec subsystem expectations. Any
feedback would be welcome.

I will wait for more comments and send a v2 in ~a week.

Kind regards,
Alvin

>
> thanks,