linux-kernel - Re: [PATCH] hid-ft260: add UART support.

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Message-ID: <Y5G2PBEprjPp3FKR@kroah.com>
Date:   Thu, 8 Dec 2022 11:02:36 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Johan Hovold <johan@...nel.org>,
        Daniel Beer <daniel.beer@...rinstitute.com>
Cc:     Jiri Kosina <jikos@...nel.org>,
        Benjamin Tissoires <benjamin.tissoires@...hat.com>,
        linux-kernel@...r.kernel.org, linux-input@...r.kernel.org,
        linux-i2c@...r.kernel.org,
        Michael Zaidman <michael.zaidman@...il.com>,
        Christina Quast <contact@...istina-quast.de>,
        linux-serial@...r.kernel.org, Jiri Slaby <jirislaby@...nel.org>
Subject: Re: [PATCH] hid-ft260: add UART support.

On Mon, Dec 05, 2022 at 02:24:03PM +1300, Daniel Beer wrote:
> On Sun, Dec 04, 2022 at 10:39:21AM +0100, Greg Kroah-Hartman wrote:
> > > Thanks for reviewing. This device is quite strange -- it presents itself
> > > as a USB HID, but it provides both an I2C master and a UART. The
> > > existing driver supports only the I2C functionality currently.
> > 
> > Lots of devices are a "fake HID" device as other operating systems make
> > it easy to write userspace drivers that way.  Linux included.  What
> > userspace programs are going to want to interact with this device and
> > what api are they going to use?
> 
> Hi Greg,
> 
> The application I'm looking at uses it as a debug console, so personally
> I'd like to be able to use it with picocom and other terminal programs.
> 
> > > > > --- a/include/uapi/linux/major.h
> > > > > +++ b/include/uapi/linux/major.h
> > > > > @@ -175,4 +175,6 @@
> > > > >  #define BLOCK_EXT_MAJOR		259
> > > > >  #define SCSI_OSD_MAJOR		260	/* open-osd's OSD scsi device */
> > > > >  
> > > > > +#define FT260_MAJOR		261
> > > > 
> > > > A whole new major for just a single tty port?  Please no, use dynamic
> > > > majors if you have to, or better yet, tie into the usb-serial
> > > > implementation (this is a USB device, right?) and then you don't have to
> > > > mess with this at all.
> > > 
> > > As far as I understand it, I don't think usb-serial is usable, due to
> > > the fact that this is already an HID driver.
> > 
> > That should not be a restriction at all.  You are adding a tty device to
> > this driver, no reason you can't interact with usb-serial instead.  That
> > way you share the correct userspace tty name and major/minor numbers and
> > all userspace tools should "just work" as they know that name and how to
> > interact with it already.
> > 
> > Try doing that instead of your own "raw" tty device please.
> 
> Maybe I've misunderstood something. The reason I thought usb-serial was
> unusable in this instance was that I couldn't see a way to create a port
> except via usb-serial's own probe function (otherwise, the API looked
> fine).
> 
> I don't know whether I'm looking at a serial or an I2C interface until
> after it's already been probed by HID core, I have a struct hid_device
> and I've asked what type of interface it is via an HID feature report.
> This can't be determined otherwise, because strapping pins affect the
> presentation of interfaces.
> 
> At that point, I (currently) call uart_add_one_port. I might have missed
> it, but I didn't see anything analogous in the usb-serial API. Am I
> going about this the wrong way?

I thought that this could be done, but I might be wrong.  Johan, any
ideas?

thanks,

greg k-h