linux-kernel - Re: [PATCH v4 7/7] usb: typec: mux: Add On Semi fsa4480 driver

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Date:   Mon, 7 Mar 2022 06:48:25 -0800
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Daniel Scally <djrscally@...il.com>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Hans de Goede <hdegoede@...hat.com>,
        linux-usb@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Subject: Re: [PATCH v4 7/7] usb: typec: mux: Add On Semi fsa4480 driver

On Mon 07 Mar 02:16 PST 2022, Andy Shevchenko wrote:

> On Sun, Mar 06, 2022 at 07:40:40PM -0800, Bjorn Andersson wrote:
> > The ON Semiconductor FSA4480 is a USB Type-C port multimedia switch with
> > support for analog audio headsets. It allows sharing a common USB Type-C
> > port to pass USB2.0 signal, analog audio, sideband use wires and analog
> > microphone signal.
> > 
> > Due to lacking upstream audio support for testing, the audio muxing is
> > left untouched, but implementation of muxing the SBU lines is provided
> > as a pair of Type-C mux and switch devices. This provides the necessary
> > support for enabling the DisplayPort altmode on devices with this
> > circuit.
> 
> ...
> 
> > +static const struct regmap_config fsa4480_regmap_config = {
> > +	.reg_bits = 8,
> > +	.val_bits = 8,
> > +	.max_register = FSA4480_MAX_REGISTER,
> > +};
> 
> You are using mutex for accessing hardware. Do you still need a regmap lock?
> If so, add a comment to explain why.
> 

I've not considered that before, but you're right, there doesn't seem to
be any reason to keep the locking in the regmap.

> ...
> 
> > +		/* 15us to allow the SBU switch to turn off */
> > +		usleep_range(15, 1000);
> 
> This is quite unusual range.
> 
> If you are fine with the long delay, why to stress the system on it?
> Otherwise the use of 1000 is unclear.
> 
> That said, I would expect one of the below:
> 
> 		usleep_range(15, 30);
> 		usleep_range(500, 1000);
> 

Glad you asked about that, as you say the typical form is to keep the
range within 2x of the lower value, or perhaps lower + 5.

But if the purpose is to specify a minimum time and then give a max to
give the system some flexibility in it's decision of when to wake up.
And in situations such as this, we're talking about someone connecting a
cable, so we're in "no rush" and I picked the completely arbitrary 1ms
as the max.

Do you see any drawback of this much higher number? (Other than it
looking "wrong")

> ...
> 
> > +	sw_desc.fwnode = dev->fwnode;
> 
> Please, don't dereference for fwnode explicitly. Use dev_fwnode().
> 

Okay, will update accordingly.

Thanks,
Bjorn

> ...
> 
> > +	mux_desc.fwnode = dev->fwnode;
> 
> Ditto.
> 
> -- 
> With Best Regards,
> Andy Shevchenko
> 
>