linux-kernel - Re: [PATCH v2 03/11] mux: core: Add usb.h header with MUX_USB_* and and MUX_TYPEC

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Date:   Fri, 8 Sep 2017 19:07:37 +0200
From:   Hans de Goede <hdegoede@...hat.com>
To:     Peter Rosin <peda@...ntia.se>,
        MyungJoo Ham <myungjoo.ham@...sung.com>,
        Chanwoo Choi <cw00.choi@...sung.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Darren Hart <dvhart@...radead.org>,
        Andy Shevchenko <andy@...radead.org>,
        Mathias Nyman <mathias.nyman@...el.com>
Cc:     linux-kernel@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        devel@...verdev.osuosl.org,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Sathyanarayanan Kuppuswamy Natarajan <sathyaosid@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org
Subject: Re: [PATCH v2 03/11] mux: core: Add usb.h header with MUX_USB_* and
 and MUX_TYPEC_* state constants

Hi,

On 08-09-17 17:47, Peter Rosin wrote:
> On 2017-09-05 18:42, Hans de Goede wrote:
>> Add MUX_USB_* and MUX_TYPEC_* state constant defines, which can be used by
>> USB device/host, resp. Type-C polarity/role/altmode mux drivers and
>> consumers to ensure that they agree on the meaning of the
>> mux_control_select() state argument.
>>
>> Signed-off-by: Hans de Goede <hdegoede@...hat.com>
>> ---
>> Changes in v2:
>> -Start numbering of defines at 0 not 1
>> -Use a new usb.h header, rather then adding these to consumer.h
>> -Add separate MUX_USB_* and MUX_TYPEC_* defines
>> ---
>>   include/linux/mux/usb.h | 32 ++++++++++++++++++++++++++++++++
>>   1 file changed, 32 insertions(+)
>>   create mode 100644 include/linux/mux/usb.h
>>
>> diff --git a/include/linux/mux/usb.h b/include/linux/mux/usb.h
>> new file mode 100644
>> index 000000000000..44df5eca5256
>> --- /dev/null
>> +++ b/include/linux/mux/usb.h
>> @@ -0,0 +1,32 @@
>> +/*
>> + * mux/usb.h - definitions for USB multiplexers
>> + *
>> + * Copyright (C) 2017 Hans de Goede <hdegoede@...hat.com>
>> + *
>> + * This program is free software; you can redistribute it and/or modify
>> + * it under the terms of the GNU General Public License version 2 as
>> + * published by the Free Software Foundation.
>> + */
>> +#ifndef _LINUX_MUX_USB_H
>> +#define _LINUX_MUX_USB_H
>> +
>> +/* Mux state values for USB device/host role muxes */
>> +#define MUX_USB_DEVICE		(0) /* USB device mode */
>> +#define MUX_USB_HOST		(1) /* USB host mode */
>> +#define MUX_USB_STATES		(2)
>> +
>> +/*
>> + * Mux state values for Type-C polarity/role/altmode muxes.
>> + *
>> + * MUX_TYPEC_POLARITY_INV may be or-ed together with any other mux-state as
>> + * inverted-polarity (Type-C plugged in upside down) can happen with any
>> + * other mux-state.
>> + */
>> +#define MUX_TYPEC_POLARITY_INV		BIT(0)   /* Polarity inverted bit */
>> +#define MUX_TYPEC_DEVICE		(0 << 1) /* USB device mode */
>> +#define MUX_TYPEC_HOST			(1 << 1) /* USB host mode */
>> +#define MUX_TYPEC_HOST_AND_DP_SRC	(2 << 1) /* USB host + 2 lanes DP src */
>> +#define MUX_TYPEC_DP_SRC		(3 << 1) /* 4 lanes Display Port src */
>> +#define MUX_TYPEC_STATES		(4 << 1)
> 
> But USB Type-C muxes need not support just these states If I read it right?
> USB Type-C seems to be usable for a variety of protocols and the above list
> seems pretty much like a special case for this mux (and perhaps a set of
> other similar muxes). But when someone with a USB Type-C mux for different
> protocols shows up, that person will probably be frustrated by these
> defines, no? Or is there something I don't see that limits USB-C to DP?

In general almost all hardware is limited to the above (+ analog audio over
the 2 Sideband use pins, but I expect that to have a separate mux).

You're right, theoretically there might be other cases, e.g. there is a spec
for HDMI over Type-C (wishful thinking from the HDMI group, no one uses this),
but:

1) I expect most muxes to implement the above set, that is what all
hardware out there supports (well that or less).

2) We can always add extra defines here, that means that a Type-C mux may
not implement all states and return -EINVAL when asked for something it
does not implement, which I understand is a bit weird from a mux subsys
pov. But that can be the case anyways because even though the mux supports
these options, the board it is used on does no necessarily have to support
these options, e.g. there may be only 2 lanes of DP hooked up to the mux
(or no DP at all, but then I would them to expect a different mux).

So the Type-C Port Manager already needs to be passed some platform
data describing which features the board has and keep that in mind
when negotiation with the dongle attached to the Type-C port, so if
we do get boards which do HDMI and no DP, then the TCPM would simply
never use the MUX_TYPEC_HOST_AND_DP_SRC and MUX_TYPEC_DP_SRC states.

Regards,

Hans