linux-kernel - Re: [PATCH 02/11] HID: i2c-hid: implement ll

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Date:	Mon, 3 Feb 2014 17:04:19 +0100
From:	David Herrmann <dh.herrmann@...il.com>
To:	Benjamin Tissoires <benjamin.tissoires@...hat.com>
Cc:	Benjamin Tissoires <benjamin.tissoires@...il.com>,
	Jiri Kosina <jkosina@...e.cz>,
	Frank Praznik <frank.praznik@...rr.com>,
	"open list:HID CORE LAYER" <linux-input@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 02/11] HID: i2c-hid: implement ll_driver transport-layer callbacks

Hi

On Sun, Feb 2, 2014 at 5:50 AM, Benjamin Tissoires
<benjamin.tissoires@...hat.com> wrote:
> Add output_report and raw_request to i2c-hid.
> Hopefully, we will manage to have the same transport level between
> all the transport drivers.
>
> Signed-off-by: Benjamin Tissoires <benjamin.tissoires@...hat.com>
> ---
>  drivers/hid/i2c-hid/i2c-hid.c | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
>
> diff --git a/drivers/hid/i2c-hid/i2c-hid.c b/drivers/hid/i2c-hid/i2c-hid.c
> index ce68a12..5099f1f 100644
> --- a/drivers/hid/i2c-hid/i2c-hid.c
> +++ b/drivers/hid/i2c-hid/i2c-hid.c
> @@ -574,6 +574,28 @@ static int i2c_hid_output_raw_report(struct hid_device *hid, __u8 *buf,
>         return ret;
>  }
>
> +static int i2c_hid_output_report(struct hid_device *hid, __u8 *buf,
> +               size_t count)
> +{
> +       return i2c_hid_output_raw_report(hid, buf, count, HID_OUTPUT_REPORT);
> +}
> +
> +static int i2c_hid_raw_request(struct hid_device *hid, unsigned char reportnum,
> +                              __u8 *buf, size_t len, unsigned char rtype,
> +                              int reqtype)
> +{
> +       switch (reqtype) {
> +       case HID_REQ_GET_REPORT:
> +               return i2c_hid_get_raw_report(hid, reportnum, buf, len, rtype);
> +       case HID_REQ_SET_REPORT:
> +               if (buf[0] != reportnum)
> +                       return -EINVAL;
> +               return i2c_hid_output_raw_report(hid, buf, len, rtype);

I just skimmed the I2C-HID specs and it defines three methods for
input/output reports:

1) Section 6.2:
raw async output-reports can be sent by writing the data at any time
to wOutputRegister.
This should be used as method for hid->output_report().

2) Section 7.1:
SET_REPORT can be issued by writing the right OPCODE + report-ID into
wCommandRegister and the data into wDataRegister.
This should be used as method for hid->raw_request() + HID_REQ_SET_REPORT.

3) Section 7.1:
GET_REPORT can be issued by writing the right OPCODE + report-ID into
wCommandRegister and then waiting for the device to write the data
into wDataRegister.
This should be used for hid->raw_request() + HID_REQ_GET_REPORT


The GET_REPORT implementation looks fine to me, but the
i2c_hid_set_report() seems to support both 1) and 2) depending on the
passed type and capabilities:
 - it uses 2) for FEATURE_REPORT reports or if the max OUTPUT_LENGTH is 0
 - it uses 1) otherwise

I'm not sure whether the i2c-hid-spec mandates this behavior, so I am
not saying it's wrong. I just wanna understand what we do here. So if
we use hid->output_report() with HID_FEATURE_REPORT, the current code
turns this into a SET_REPORT. Likewise, an hid->raw_request() with
HID_REQ_SET_REPORT but with HID_OUTPUT_REPORT turns into an
output-report.

I'd rather expect this behavior:

hid->output_report() should always do this:
  args[index++] = outputRegister & 0xFF;
  args[index++] = outputRegister >> 8;
  hidcmd = &hid_no_cmd;

while hid->raw_request() should always do this:
  args[index++] = dataRegister & 0xFF;
  args[index++] = dataRegister >> 8;

The special case for maxOutputLength==0 seems fine to me, but the
"reportType == 0x03" looks weird.


Thanks
David

> +       default:
> +               return -EIO;
> +       }
> +}
> +
>  static void i2c_hid_request(struct hid_device *hid, struct hid_report *rep,
>                 int reqtype)
>  {
> @@ -761,6 +783,8 @@ static struct hid_ll_driver i2c_hid_ll_driver = {
>         .close = i2c_hid_close,
>         .power = i2c_hid_power,
>         .request = i2c_hid_request,
> +       .output_report = i2c_hid_output_report,
> +       .raw_request = i2c_hid_raw_request,
>  };
>
>  static int i2c_hid_init_irq(struct i2c_client *client)
> --
> 1.8.3.1
>
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