linux-kernel - Re: [PATCH] Add support for usbfs zerocopy.

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Message-ID: <20160106153601.GA25632@imap.gmail.com>
Date:	Wed, 6 Jan 2016 16:36:01 +0100
From:	"Steinar H. Gunderson" <sesse@...gle.com>
To:	Peter Stuge <peter@...ge.se>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
	stern@...land.harvard.edu
Subject: Re: [PATCH] Add support for usbfs zerocopy.

On Wed, Jan 06, 2016 at 04:22:12PM +0100, Peter Stuge wrote:
>>> Our interface for zero copy reads/writes is O_DIRECT, and that requires
>>> not special memory allocation, just proper alignment.
>> But that assumes you are using I/O using read()/write(). There's no way you
>> can shoehorn USB isochronous reads into the read() interface, O_DIRECT or not.
> How about aio?

I don't really see how; a USB device does not look much like a file. (Where
would you stick the endpoint, for one? And how would you ever submit an URB
with multiple packets in it, which is essential?) It feels a bit like
trying to use UDP sockets with only read() and write().

In any case, the usbfs interface already exists and is stable. This is about
extending it; replacing it with something new from scratch to get zerocopy
would seem overkill.

/* Steinar */
-- 
Software Engineer, Google Switzerland
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