Re: [PATCH 1/1] Don't Send Feature Reports on Interrupt Endpoint

[Alan Stern]

On Wed, 15 Sep 2010, Alan Ott wrote:

> On 09/15/2010 12:10 PM, Alan Stern wrote:
> > On Wed, 15 Sep 2010, Alan Ott wrote:
> >
> >    
> >> Feature reports should only be sent on the control endpoint.
> >>      
> > Where is this requirement?  Section 5.6 of the HID spec says:
> >
> > 	Note  Only Input reports are sent via the Interrupt In pipe.
> > 	Feature and Output reports must be initiated by the host via
> > 	the Control pipe or an optional Interrupt Out pipe.
> >
> > So if there is an Interrupt-OUT endpoint, it should be valid to use it
> > for a Feature report.
> >
> > Alan Stern
> >    
> 
> That is true, the standard does say that. Thanks for checking my work.
> 
> I expected to go into the HID standard and find something that would 
> back me up. All I ended up finding was contradictions. I know I had seen 
> it somewhere (that Feature reports must use the Control endpoint), and 
> eventually I remembered that I had read it in Jan Axelson's USB 
> Complete, 3rd Edition[1]. I sent her an email asking what she based it 
> on, and I cited parts of the HID standard which I thought to be either 
> unspecific, or even seemed to indicate the opposite of what she asserts 
> in her book (one good example of which is the one you cited). I have not 
> heard back from her yet (but she does indeed answer her email, so I 
> expect something in a day or two).
> 
> In addition to the section you cited, there's also section 6.2.2.5 which 
> says, at the very end of the section, on page 32:
> 
>     Output type reports can optionally be sent via an Interrupt Out pipe.
>     While similar in function, Output and Feature items differ in the
>     following
>     ways:
>          [snip]
>        Like Output items, Feature items make up Feature Reports
>     accessible via the Control
>        pipe with the Get_Report (Feature) and Set_Report (Feature) requests.
> 
> That section seems to say both ways, depending on how you read "Output 
> type" (ie: does it mean OUTPUT reports or does it mean "reports which go 
> out from the host"). Note that particular section is the only place 
> where the wording "input type" or "output type" is used, indicating it 
> may mean "reports which go out from the host."
> 
> The second part of that quote says feature reports are accessible 
> through the control pipe. (It doesn't say that they _aren't_ accessible 
> any other way). It's curious to me why it would say it in that way, 
> without saying "only accessible" or "also accessible."
> 
> In all, although somewhat unclear, the HID document does seem to suggest 
> that the Interrupt OUT pipe can handle Feature Reports. However, there 
> are several things which make me think otherwise.
> 
> 1. The Windows implementation will refuse to send feature reports 
> through WriteFile() (which is the function used to send reports out the 
> interrupt OUT pipe if it exists). The HidD_SetFeature() function will 
> ONLY send feature reports out the Control pipe, regardless of the 
> presence of an Interrupt OUT endpoint.
> 
> 2. The Macintosh HID implementation does the same thing as the Windows 
> version. On the Mac, there's IOHIDDeviceSetReport() which allows you to 
> specify a report type of Feature or Output. Output reports go to the 
> Interrupt OUT pipe if it exists; feature reports do not. Feature reports 
> only go out the Control endpoint.
> 
> 3. Jan Axelson's book. While it's not an official standard, it's widely 
> accepted as a good general reference on USB. Like I said, I have an 
> email in to the author asking her to cite her source on Feature Reports. 
> That said, the book is very Windows-centric on the host side, and she 
> may be basing her assertion on #1 above.
> 
> 4. Simon (Mungewell)'s email which started this whole thing. First, he 
> indicates that his device doesn't handle Feature reports in the 
> Interrupt OUT pipe. Second, he asserts that libhid works the same way 
> Windows and Mac do, sending Feature reports out the Control pipe[2]. I 
> have a PS3 controller that I borrowed which also seems to work the same 
> way (ie: Feature reports don't work if they go out the Interrupt OUT 
> endpoint).
> 
> 5. The Bluetooth HID specification says in multiple places that "Feature 
> Reports must be carried on the Control channel." Yes yes, you don't have 
> to say it. Bluetooth is not USB, so the bluetooth standard shouldn't 
> apply. Maybe true.
> 
> So there it is. That's everything I know about this particular problem. 
> The standard to me is unclear at best; Every single other 
> implementation[3] (Windows, Mac, libhid) uses the control endpoint only; 
> Jan's book says that only the control endpoint can be used; Simon's 
> hardware only works using the control endpoint for feature reports; my 
> hardware only works using the control endpoint for Feature reports; the 
> Bluetooth spec says Feature reports must use the control endpoint.
> 
> PS: I'll throw out one more thing. HID transfers which use the control 
> endpoint have their report type identified in the wValue field of their 
> header. Transfers which use the Interrupt OUT endpoint do not have their 
> type identified in any way. (they only have the report ID, and even 
> then, only when numbered reports are used). If a device which did not 
> use report IDs had both a single OUTPUT and a single FEATURE report, how 
> could the reports be differentiated by the device if it were possible 
> for the FEATURE report to go out on the OUT endpoint? I think that 
> actually closes the argument in my mind.
> 
> I'd be happy to hear alternate theories.

That's all fine, and I have no objection to the patch itself.  You 
should include (in brief form) some of this explanation in the patch 
description so that people will know _why_ this has to be done.

Alan Stern

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