linux-kernel - Re: [PATCH] usb:typec:tcpm:support double Rp to Vbus cable as sink

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Message-ID: <c52acbd0-e8e3-83e7-d87b-939b47aa39c1@roeck-us.net>
Date:   Mon, 18 Sep 2023 07:22:41 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Michael Wu <michael@...winnertech.com>
Cc:     gregkh@...uxfoundation.org, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] usb:typec:tcpm:support double Rp to Vbus cable as sink

On 9/18/23 03:31, Heikki Krogerus wrote:
> On Thu, Sep 14, 2023 at 08:31:54AM +0800, Michael Wu wrote:
>> The USB Type-C Cable and Connector Specification defines the wire
>> connections for the USB Type-C to USB 2.0 Standard-A cable assembly
>> (Release 2.2, Chapter 3.5.2).
>> The Notes says that Pin A5 (CC) of the USB Type-C plug shall be connected
>> to Vbus through a resister Rp.
>> However, there is a large amount of such double Rp connected to Vbus
>> non-standard cables which produced by UGREEN circulating on the market, and
>> it can affects the normal operations of the state machine easily,
>> especially to CC1 and CC2 be pulled up at the same time.
>> In fact, we can regard those cables as sink to avoid abnormal state.
>>
>> Message as follow:
>> [   58.900212] VBUS on
>> [   59.265433] CC1: 0 -> 3, CC2: 0 -> 3 [state TOGGLING, polarity 0, connected]
>> [   62.623308] CC1: 3 -> 0, CC2: 3 -> 0 [state TOGGLING, polarity 0, disconnected]
>> [   62.625006] VBUS off
>> [   62.625012] VBUS VSAFE0V
>>
>> Signed-off-by: Michael Wu <michael@...winnertech.com>
>> ---
>>   drivers/usb/typec/tcpm/tcpm.c | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/usb/typec/tcpm/tcpm.c b/drivers/usb/typec/tcpm/tcpm.c
>> index d962f67c95ae6..beb7143128667 100644
>> --- a/drivers/usb/typec/tcpm/tcpm.c
>> +++ b/drivers/usb/typec/tcpm/tcpm.c
>> @@ -519,7 +519,8 @@ static const char * const pd_rev[] = {
>>   
>>   #define tcpm_port_is_sink(port) \
>>   	((tcpm_cc_is_sink((port)->cc1) && !tcpm_cc_is_sink((port)->cc2)) || \
>> -	 (tcpm_cc_is_sink((port)->cc2) && !tcpm_cc_is_sink((port)->cc1)))
>> +	 (tcpm_cc_is_sink((port)->cc2) && !tcpm_cc_is_sink((port)->cc1)) || \
>> +	 (tcpm_cc_is_sink((port)->cc1) && tcpm_cc_is_sink((port)->cc2)))
>>   
>>   #define tcpm_cc_is_source(cc) ((cc) == TYPEC_CC_RD)
>>   #define tcpm_cc_is_audio(cc) ((cc) == TYPEC_CC_RA)
> 
> This look OK to me, but I would still like to wait for comments from
> Guenter - just in case.
> 

Look at the conditions. Reordered, we end up with
	(tcpm_cc_is_sink((port)->cc1) && !tcpm_cc_is_sink((port)->cc2)) ||
	(tcpm_cc_is_sink((port)->cc1) && tcpm_cc_is_sink((port)->cc2))
which simplifies to
	tcpm_cc_is_sink((port)->cc1)
making the complete expression
	tcpm_cc_is_sink((port)->cc1) ||
	(tcpm_cc_is_sink((port)->cc2) && !tcpm_cc_is_sink((port)->cc1))
which simplifies further to
	tcpm_cc_is_sink((port)->cc1) || tcpm_cc_is_sink((port)->cc2)

The simplified expression doesn't conflict with other detections, so I am
ok with it. It might be worthwhile adding a comment to the code, though,
explaining the reason.

Guenter

> thanks,
>