linux-kernel - Re: [PATCH v5 7/7] usb: gadget: f

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Message-ID: <5658B46F.6030601@ladisch.de>
Date:	Fri, 27 Nov 2015 20:52:15 +0100
From:	Clemens Ladisch <clemens@...isch.de>
To:	Felipe Ferreri Tonello <eu@...ipetonello.com>,
	linux-usb@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, Felipe Balbi <balbi@...com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Robert Baldyga <r.baldyga@...sung.com>
Subject: Re: [PATCH v5 7/7] usb: gadget: f_midi: pre-allocate IN requests

Felipe Ferreri Tonello wrote:
> One thing to consider is that the ALSA rawmidi device buffer is
> sequential and our USB request buffer is not. This means that our 32
> (qlen) * 256 (buflen) = 8KB of data is non-linear. Some requests might
> have 3 or 4 bytes (average size of a normal MIDI message) of data and
> some others might contain the full 256 bytes (for SysEx messages).

f_midi_transmit() always fills up the USB packet as much as possible, so
the number of MIDI messages per request will increase automatically when
the ALSA buffer fills up faster that it is emptied by f_midi.

> == Conclusion ==
>
> Based on our conversation and your suggestions, I think that to just
> ignore if an overrun occurs to the USB requests is fine. Upon completion
> the request will be reused.
> Important to note that if the overrun occurs, it will cause user-space
> to block until a) the completion function is called successfully or b)
> snd_rawmidi_write() times out. Which I think this is expected by ALSA users.
>
> Does that make sense?

Yes.


Regards,
Clemens
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