Hi Geert-san, > From: geert.uytterhoeven@xxxxxxxxx > 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@xxxxxxxxxxx> 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@xxxxxxxxxxx> > > +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@xxxxxxxxxxxxxx > > 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
