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

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Message-ID: <20160106144512.GA21737@imap.gmail.com>
Date:	Wed, 6 Jan 2016 15:45:12 +0100
From:	"Steinar H. Gunderson" <sesse@...gle.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	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 Tue, Jan 05, 2016 at 10:49:49PM -0800, Christoph Hellwig wrote:
> This is a completely broken usage of the mmap interface.  if you use
> mmap on a device file you must use the actual mmap for the data
> transfer.

Really? V4L does exactly the same thing, from what I can see. It's just a way
of allocating memory with specific properties, roughly similar to hugetlbfs.

> 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.

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