From: Alan Stern <stern@rowland.harvard.edu>
To: Sid Spry <sid@aeam.us>
Cc: linux-usb@vger.kernel.org
Subject: Re: Unable to Use Isochronous Behavior w/ Isoc Endpoint in FunctionFC
Date: Sun, 21 Jun 2020 10:09:16 -0400 [thread overview]
Message-ID: <20200621140916.GB107361@rowland.harvard.edu> (raw)
In-Reply-To: <dc61359f-2a04-4590-9ac9-81b4f6e8d3b8@www.fastmail.com>
On Sat, Jun 20, 2020 at 10:38:33PM -0500, Sid Spry wrote:
> When I was using a repurposed audio device I had to set an alternate mode. Is
> that related to the issue here? The alternate mode seems to be a relic of the
> descriptor layout before I dropped the device class and substituted my own
> driver. The current descriptors specify no alternate modes.
I don't know much about FunctionFS, so I can't help with your main
question about isochronous packets not being dropped. But I can explain
this.
In the USB-2.0 spec, at the end of section 5.6.3 the text says:
All device default interface settings must not include any
isochronous endpoints with non-zero data payload sizes (specified
via wMaxPacketSize in the endpoint descriptor). Alternate
interface settings may specify non-zero data payload sizes for
isochronous endpoints.
That's why you had to select an alternate setting before transferring
any isochronous data. Any isochronous endpoint in altsetting 0 must have
its maxpacket size set to 0.
Alan Stern
next prev parent reply other threads:[~2020-06-21 14:09 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-21 3:38 Unable to Use Isochronous Behavior w/ Isoc Endpoint in FunctionFC Sid Spry
2020-06-21 14:09 ` Alan Stern [this message]
2020-06-22 2:25 ` Sid Spry
2020-06-22 14:02 ` Alan Stern
2020-06-22 15:41 ` Sid Spry
2020-06-22 16:41 ` Alan Stern
2020-06-22 21:13 ` Sid Spry
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