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SubjectRe: [PATCH] phy: rcar-gen3-usb2: add sysfs for usb role swap
Hi Shimoda-san,

On Tue, Oct 18, 2016 at 8:19 AM, Yoshihiro Shimoda
<yoshihiro.shimoda.uh@renesas.com> wrote:
>> From: geert.uytterhoeven@gmail.com
>> Sent: Monday, October 17, 2016 9:28 PM
>> On Mon, Oct 17, 2016 at 9:10 AM, Yoshihiro Shimoda
>> <yoshihiro.shimoda.uh@renesas.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@renesas.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.

Thanks for the explanation!

Regardless, I think it would be good to CC the USB maintainer and mailing
list on next submission.

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-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

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