linux-kernel - Re: [PATCH v3 03/11] usb: typec: Add plug num

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Message-ID: <20201118120408.GL3437448@kuha.fi.intel.com>
Date:   Wed, 18 Nov 2020 14:04:08 +0200
From:   Heikki Krogerus <heikki.krogerus@...ux.intel.com>
To:     Prashant Malani <pmalani@...omium.org>
Cc:     linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org,
        gregkh@...uxfoundation.org, enric.balletbo@...labora.com
Subject: Re: [PATCH v3 03/11] usb: typec: Add plug num_altmodes sysfs attr

On Tue, Nov 17, 2020 at 09:40:16AM -0800, Prashant Malani wrote:
> Hi Heikki,
> 
> On Tue, Nov 17, 2020 at 02:41:43PM +0200, Heikki Krogerus wrote:
> > On Mon, Nov 16, 2020 at 12:11:42PM -0800, Prashant Malani wrote:
> > > Add a field to the typec_plug struct to record the number of available
> > > altmodes as well as the corresponding sysfs attribute to expose this to
> > > userspace.
> > > 
> > > This allows userspace to determine whether there are any
> > > remaining alternate modes left to be registered by the kernel driver. It
> > > can begin executing any policy state machine after all available
> > > alternate modes have been registered with the connector class framework.
> > > 
> > > This value is set to "-1" initially, signifying that a valid number of
> > > alternate modes haven't been set for the plug. The sysfs file remains
> > > hidden as long as the attribute value is -1.
> > 
> > Why couldn't we just keep it hidden for as long as the number of
> > alt modes is 0? If you already explained that, then I apologise, I've
> > forgotten.
> > 
> 
> No worries :)
> 
> Succinctly, because 0 is a valid value for "number of altmodes
> supported".
> 
> If we keep the attribute hidden for 0, then there won't
> be a way for userspace to determine that PD discovery is done and we
> don't expect any more cable plug altmodes to be registered by the kernel
> Type C port driver (it can determine this by comparing
> "number_of_altmodes" against the actual number of alt modes registered
> by the Type C port driver).
> 
> If we keep "number_of_altmodes" hidden even for 0, the userspace cannot
> differentiate between "this cable doesn't support any altmodes" and
> "it does altmodes, but the PD stack hasn't completed PD Discovery
> including DiscoverIdentity yet".
> 
> For reference, here is the initial patch and mini-discussion around it
> back in July for port-partner altmodes [1] (I've followed a similar
> logic here).
> 
> Hope this helps the rationale a bit more.

Got it. Thanks for the explanation (again :-).

Reviewed-by: Heikki Krogerus <heikki.krogerus@...ux.intel.com>

> Best regards,
> 
> -Prashant
> 
> [1]:
> https://lore.kernel.org/linux-usb/20200701082230.GF856968@kuha.fi.intel.com/

thanks,

-- 
heikki