From: Douglas Gilbert <dgilbert@interlog.com>
To: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Cc: Benson Leung <bleung@google.com>,
Prashant Malani <pmalani@chromium.org>,
Badhri Jagan Sridharan <badhri@google.com>,
Guenter Roeck <linux@roeck-us.net>,
Saranya Gopal <saranya.gopal@intel.com>,
linux-usb@vger.kernel.org
Subject: Re: [typec] pd: fixed source capability: peak_current field
Date: Mon, 11 Sep 2023 12:45:42 -0400 [thread overview]
Message-ID: <b503eb21-3850-0072-65b3-977be98e58d0@interlog.com> (raw)
In-Reply-To: <ZP8Z+1Kn+RagnFNq@kuha.fi.intel.com>
On 2023-09-11 09:45, Heikki Krogerus wrote:
> +linux-usb ml, Guenter and others.
>
> On Sun, Sep 10, 2023 at 09:29:04PM -0400, Douglas Gilbert wrote:
>> Hi,
>> The Peak current 2 bit field has been present in the fixed source PDO
>> since PD 2.0 . It is commented out in typec/pd.c with a note to revisit
>> as it needs the Operational current field from the active RDO.
>>
>> Not sure I agree with that: the 2 bit field is unit-less and could easily
>> be displayed as such. At the moment the active RDO is not directly
>> available through sysfs and would certainly be a useful addition for
>> many reasons.
>>
>> So my vote (not that I get one) would be to output peak_current in
>> sysfs with no unit. Then in the ABI peak_current could be explained
>> by reference to a table in the spec (e.g. "USB Power Delivery
>> Specification Revision 3.1, Version 1.8, table 6-10 Fixed Power
>> Source Peak Current Capability"). Then the issue of what it means
>> is handed off to the user space :-)
>>
>> Also of note is that the Peak current field is B21..B20 which is
>> below B23 (obviously) so all fixed source PDOs have that field
>> (and thus those unit-less values could differ), not just
>> PDO #1 .
>
> I agree with you. I don't think it makes sense to try to interpret the
> peak current value in kernel, so we could just expose the value as-is.
>
> But first giving the others heads-up on this, just in case somebody
> disagrees.
Similar situation with fast_role_swap in the fixed sink PDO. It is not really a
current but an index to a table embedded in the r31_v18's table 6-16.
Importantly the value 0 implies the sink does not support FRS. So again it
seems to be unit-less so the suffix "_current" in its sysfs name is
questionable. The spec does help much here formally naming it: the "Fast Role
Swap USB Type-C® Current" field in section 6.4.1.3.1.6 . The show function
for the fast_role_swap_current is defined in typec/pd.c but I didn't see any
invocations.
Doug Gilbert
prev parent reply other threads:[~2023-09-11 21:41 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <9b87f828-cf1d-e370-f4ca-e0c53c9a35c5@interlog.com>
2023-09-11 13:45 ` [typec] pd: fixed source capability: peak_current field Heikki Krogerus
2023-09-11 16:45 ` Douglas Gilbert [this message]
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