linux-kernel - Re: [PATCH 2/3] HID: steam: add serial number information.

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Date:   Fri, 16 Feb 2018 10:57:22 +0100
From:   Rodrigo Rivas Costa <rodrigorivascosta@...il.com>
To:     Benjamin Tissoires <benjamin.tissoires@...hat.com>
Cc:     Jiri Kosina <jikos@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>, linux-input@...r.kernel.org
Subject: Re: [PATCH 2/3] HID: steam: add serial number information.

On Fri, Feb 16, 2018 at 10:31:35AM +0100, Benjamin Tissoires wrote:
> > Ok, I'll do that. The weird thing, however, is that:
> >
> >         hid_hw_raw_request(steam->hid_dev, 0x00,
> >                 buf, hid_report_len(r), /* 64 */
> >                 HID_FEATURE_REPORT, HID_REQ_GET_REPORT);
> >
> > fails with EOVERFLOW. I have to use:
> >
> >         hid_hw_raw_request(steam->hid_dev, 0x00,
> >                 buf, 65
> >                 HID_FEATURE_REPORT, HID_REQ_GET_REPORT);
> >
> > which just feels wrong to me.
> 
> Indeed
> 
> >
> > And looking around drivers/hid/*.c I see that most calls to
> > hid_hw_raw_request(..., HID_REQ_GET_REPORT) use a buffer allocated with
> > {devm_,}kzalloc() and a constant length, never using
> > hid_alloc_report_buf() or hid_report_len().
> 
> well, hid-input.c and hid-multitouch.c are using
> hid_alloc_report_buf() and these two are the most generic ones. We
> haven't converted everybody to use hid_alloc_report_buf(), but it's
> not a reason to not use it for new drivers :)

Agreed. And they use hid_report_len() on the hid_hw_raw_request().

Now my guess is that hid_report_len() should return 65 instead of 64.
And it would do that if the report.id would be >0, but, alas, it is =0.
Maybe feature reports should add 1 without checking id>0? Or maybe
the Steam report descriptor is wrong and I should use report_fixup or
something?

> No, it's unlikely that there is a bug. Can you forward to me the
> report descriptors of the Steam controller (ideally with the tool
> hid-recorder from http://bentiss.github.io/hid-replay-docs/, so I can
> get a few events too)?

Sure, see the attached file. It is the wired one, I opened jstest and
moved the joystick around to get a few events. Unfortunately the
HID_REQ_GET_REPORT are not recorded.

For the casual reader, here is the decoded report descriptor [1]:

0x06, 0x00, 0xFF,  // Usage Page (Vendor Defined 0xFF00)
0x09, 0x01,        // Usage (0x01)
0xA1, 0x01,        // Collection (Application)
0x15, 0x00,        //   Logical Minimum (0)
0x26, 0xFF, 0x00,  //   Logical Maximum (255)
0x75, 0x08,        //   Report Size (8)
0x95, 0x40,        //   Report Count (64)
0x09, 0x01,        //   Usage (0x01)
0x81, 0x02,        //   Input (Data,Var,Abs,No Wrap,Linear,Preferred State,No Null Position)
0x95, 0x40,        //   Report Count (64)
0x09, 0x01,        //   Usage (0x01)
0x91, 0x02,        //   Output (Data,Var,Abs,No Wrap,Linear,Preferred State,No Null Position,Non-volatile)
0x95, 0x40,        //   Report Count (64)
0x09, 0x01,        //   Usage (0x01)
0xB1, 0x02,        //   Feature (Data,Var,Abs,No Wrap,Linear,Preferred State,No Null Position,Non-volatile)
0xC0,              // End Collection


Regards.
Rodrigo

[1]: http://eleccelerator.com/usbdescreqparser/

View attachment "steam-hid.txt" of type "text/plain" (7442 bytes)