Re: usb: typec: tcpm: type-mismatched PDO handling in tcpm_pd_build_request()

[Heikki Krogerus <heikki.krogerus@linux.intel.com>]

+Badhri, and also RD and Amit

On Sun, Sep 20, 2026 at 05:29:35PM -0400, Seungjin Bae wrote:
> Hi,
> 
> I found that tcpm_pd_select_pdo() can match a source and sink PDO of
> different types. Before sending a fix, I would like to ask whether
> this type-mixed match is intended, and if not, whether you would
> prefer a same-type check in tcpm_pd_select_pdo() or explicit type
> handling in tcpm_pd_build_request().

I think a patch would have been fine. You can always mark it as RFC if
uncertain.

Thanks,

> The tcpm_pd_select_pdo() function matches a source PDO against a sink
> PDO using their voltage ranges only, without checking that the two
> PDOs are of the same type. A source PDO and a sink PDO of different
> types (e.g. a Battery source PDO and a Fixed sink PDO) can therefore
> be matched as long as their voltage ranges overlap.
> 
> tcpm_pd_build_request() then combines the matched pair with
> min_power()/min_current(), which apply the same accessor to both
> operands. pdo_max_current() and pdo_max_power() decode the same bits
> (9:0) of the PDO but scale them differently (x10 mA vs x250 mW), so
> when the matched types differ, the sink operand is decoded with the
> wrong accessor. This misreads the sink's capability and weakens the
> min() bound intended to cap the request to the sink's limit.
> 
> This type-mixed match became possible after commit 53fe0de9a35d
> ("usb: typec: tcpm: pdo matching optimization") relaxed the match to
> voltage range only; the min_power()/min_current() macros still assume
> a matched pair shares the same type.
> 
> I have not observed this on real hardware; I found it by static
> analysis.

-- 
heikki