linux-kernel - Re: [PATCH v3 1/4] usb: gadget: f_midi: free usb request when done

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Date:	Mon, 12 Oct 2015 14:10:14 +0200
From:	Clemens Ladisch <clemens@...isch.de>
To:	Felipe Tonello <eu@...ipetonello.com>
Cc:	USB list <linux-usb@...r.kernel.org>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	Peter Chen <Peter.Chen@...escale.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Felipe Balbi <balbi@...com>,
	Andrzej Pietrasiewicz <andrzej.p@...sung.com>
Subject: Re: [PATCH v3 1/4] usb: gadget: f_midi: free usb request when done

Felipe Tonello wrote:
> On Mon, Oct 12, 2015 at 11:16 AM, Clemens Ladisch <clemens@...isch.de> wrote:
>> Felipe Tonello wrote:
>>> I believe that is the best way to implement. Create multiple requests
>>> until the ALSA substreams buffer are empty and free the request on
>>> completion.
>>
>> I believe a better way to implement this is to allocate a fixed number
>> of requests, and to always reuse them.
>
> How many?

Enough to get proper pipelining.  At least two, maybe not more.
(Depends on how fast those tiny CPUs can queue the next request.)

>>> The problem of having requests when host isn't listening will happen
>>> anyway because there is no way to know that until completion.
>>
>> But if you have no upper limit on the number of queues requests, you
>> will eventually run out of (DMA) memory.
>
> And that's what happening at the moment. One of my patches are to fix
> a memory leak when that happens.
>
> But it would be ideal to have a FIFO of requests and perhaps ignore
> new requests if the FIFO is full.
>
> So, allocate (pre-allocate?) new requests until the FIFO is full. Upon
> completion, remove the request from FIFO, but still reuse it on
> f_midi_transmit() and queue it on the FIFO again if there is still
> data from ALSA, otherwise just free the request.

Yes, that's exactly what I'm proposing.


Regards,
Clemens
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