linux-kernel - Re: [RFC PATCH 1/3] usb: pd: Increase max PDO objects to 13

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Date:   Fri, 3 Sep 2021 11:05:07 -0700
From:   Jack Pham <jackp@...eaurora.org>
To:     Prashant Malani <pmalani@...omium.org>
Cc:     linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org,
        linux-pm@...r.kernel.org, bleung@...omium.org,
        heikki.krogerus@...ux.intel.com, badhri@...gle.com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Sebastian Reichel <sre@...nel.org>
Subject: Re: [RFC PATCH 1/3] usb: pd: Increase max PDO objects to 13

On Thu, Sep 02, 2021 at 11:47:01PM -0700, Jack Pham wrote:
> Hi Prashant,
> 
> On Thu, Sep 02, 2021 at 02:34:58PM -0700, Prashant Malani wrote:
> > Increase the max number of PDO objects to 13, to accommodate the extra
> > PDOs added as a part of EPR (Extended Power Range) operation introduced
> > in the USB PD Spec Rev 3.1, v 1.0. See Figure 6-54 for details.
> > 
> > Signed-off-by: Prashant Malani <pmalani@...omium.org>
> > ---
> >  include/linux/usb/pd.h | 8 +++++++-
> >  1 file changed, 7 insertions(+), 1 deletion(-)
> > 
> > diff --git a/include/linux/usb/pd.h b/include/linux/usb/pd.h
> > index 96b7ff66f074..7e8bdca1ce6e 100644
> > --- a/include/linux/usb/pd.h
> > +++ b/include/linux/usb/pd.h
> > @@ -201,7 +201,13 @@ struct pd_message {
> >  } __packed;
> >  
> >  /* PDO: Power Data Object */
> > -#define PDO_MAX_OBJECTS		7
> > +
> > +/*
> > + * The EPR (Extended Power Range) structure is a superset of the SPR (Standard Power Range)
> > + * capabilities structure, so set the max number of PDOs to 13 instead of 7. On SPR-only systems,
> > + * objects 8 through 13 will just be empty.
> > + */
> > +#define PDO_MAX_OBJECTS		13
> 
> Hmm this might break the recent change I made to UCSI in commit
> 1f4642b72be7 ("usb: typec: ucsi: Retrieve all the PDOs instead of just
> the first 4").
> 
>  520 static void ucsi_get_src_pdos(struct ucsi_connector *con, int is_partner)
>  521 {
>  522         int ret;
>  523
>  524         /* UCSI max payload means only getting at most 4 PDOs at a time */
>  525         ret = ucsi_get_pdos(con, 1, con->src_pdos, 0, UCSI_MAX_PDOS);
>  526         if (ret < 0)
>  527                 return;
>  528
>  529         con->num_pdos = ret / sizeof(u32); /* number of bytes to 32-bit PDOs */
>  530         if (con->num_pdos < UCSI_MAX_PDOS)
>  531                 return;
>  532
>  533         /* get the remaining PDOs, if any */
>  534         ret = ucsi_get_pdos(con, 1, con->src_pdos, UCSI_MAX_PDOS,
>  535                             PDO_MAX_OBJECTS - UCSI_MAX_PDOS);
> 				 ^^^^^^^^^^^^^^^
> This routine calls the UCSI GET_PDOS command for up to 4 PDOs at a time
> since that's the most the return payload can carry.  Currently this
> assumes that we'd only need to request the PPM at most twice to retrieve
> all the PDOs for up to a maximum of 7 (first request for 4 then again if
> needed for the remaining 3).  I'm not sure if any existing UCSI FW would
> be updatable to support more than 7 PDOs in the future, much less
> support EPR.  In fact, current UCSI 1.2 spec [1] Table 4-34 mentions PDO

Sorry, forgot the footnote with the link to the spec:
[1] https://www.intel.com/content/dam/www/public/us/en/documents/technical-specifications/usb-type-c-ucsi-spec.pdf

> offset valid values are 0-7 and anything else "shall not be used", so I
> don't know how UCSI will eventually cope with EPR without a spec update.
> 
> So if this macro changes to 13 then this call would result in a call to
> the UCSI GET_PDOS command passing num_pdos == 13-4 = 9 which would
> probably result in an error from the PPM FW.  So we might need to retain
> the maximum value of 7 PDOs at least for UCSI here.  Maybe that means
> this UCSI driver needs to carry its own definition of
> UCSI_MAX_TOTAL_PDOS=7 instead of using PDO_MAX_OBJECTS?
> 
> Jack
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