linux-kernel - RE: [PATCH] phy: rcar-gen3-usb2: add sysfs for usb role swap

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Date:   Tue, 18 Oct 2016 06:19:04 +0000
From:   Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
CC:     Kishon Vijay Abraham I <kishon@...com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>
Subject: RE: [PATCH] phy: rcar-gen3-usb2: add sysfs for usb role swap

Hi Geert-san,

> From: geert.uytterhoeven@...il.com
> Sent: Monday, October 17, 2016 9:28 PM
> 
> Hi Shimoda-san,
> 
> On Mon, Oct 17, 2016 at 9:10 AM, Yoshihiro Shimoda
> <yoshihiro.shimoda.uh@...esas.com> wrote:
> > This patch adds sysfs "otg_inputs" for usb role swap. This parameter
> > is write-only and if you use them as the following, you can swap
> > the usb role.
> 
> Thank you for your patch!

Thank you for the review!

> > For example:
> >  1) connect a usb cable using 2 salvator-x boards
> >  2) On A-device (as host), you input the following command:
> >    # echo a_bus_req/ > /sys/devices/platform/soc/ee080200.usb-phy/otg_inputs
> >  3) On B-device (as peripheral), you input the following command:
> >    # echo b_bus_req > /sys/devices/platform/soc/ee080200.usb-phy/otg_inputs
> 
> At first, I thought the trailing "/" was a typo...
> 
> > --- /dev/null
> > +++ b/Documentation/ABI/testing/sysfs-platform-phy-rcar-gen3-usb2
> > @@ -0,0 +1,11 @@
> > +What:          /sys/devices/platform/<phy-name>/otg-inputs
> > +Date:          October 2016
> > +KernelVersion: 4.10
> > +Contact:       Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>
> > +Description:
> > +               This write-only file changes the phy mode for role swap of usb.
> > +               This file accepts the following strings:
> > +                "a_bus_req/" - switching from A-Host to A-Peripheral
> > +                "a_bus_drop" - switching from A-Peripheral to A-Host
> > +                "b_bus_req"  - switching from B-Peripheral to B-Host
> > +                "b_bus_req/" - switching from B-Host to B-Peripheral
> 
> ... until I read the above.
> 
> What's the rationale of doing it like this? I.e.
>   1. Why differentiate by trailing "/"?

This usb role swap feature is not compatible with USB OTG though,
but this strings are from the manual that usb.org released:
http://www.usb.org/developers/docs/usb20_docs/
 --> usb_20_091216.zip
  --> USB OTG and Embedded Host/USB_OTG_and_EH_2-0-version 1_1a.pdf
   Please refer to the following figures:
    - Figure 7-1: OTG A-device with HNP State Diagram
    - Figure 7-4: OTG B-device with HNP State Diagram

So, it seems the "/" means FALSE.

>   2. Why the asymmetry ("a_bus_drop" vs. "a_bus_req")?

This is also related the document.
 - From "a_host" to "a_suspend" state if "a_bus_req/" event happens.
 - From "a_suspend" to "a_host" state if "a_bus_req" event happens.
  (But the driver doesn't support it and we can use "a_bus_drop".)
 - From "a_peripheral" to "a_wait_vfall" state if "a_bus_drop" event happens.

> I do not really follow USB development, so I please accepty my apologies if
> I missed the discussion and valid arguments that lead to this.
> 
> I did find Documentation/ABI/testing/sysfs-platform-chipidea-usb-otg,
> which uses similar naming, but a slightly different mechanism (multiple
> sysfs virtual files with 0/1 states).

I also did look at the document. But, I prefer one parameter to control it.

Best regards,
Yoshihiro Shimoda

> Thanks!
> 
> Gr{oetje,eeting}s,
> 
>                         Geert
> 
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org
> 
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
>                                 -- Linus Torvalds