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

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Message-ID: <Pine.LNX.4.44L0.1601061032000.1579-100000@iolanthe.rowland.org>
Date:	Wed, 6 Jan 2016 10:35:05 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	"Steinar H. Gunderson" <sesse@...gle.com>
cc:	Christoph Hellwig <hch@...radead.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	<linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Add support for usbfs zerocopy.

On Wed, 6 Jan 2016, Steinar H. Gunderson wrote:

> 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.

Indeed, the I/O operations we are using with mmap here are not reads or 
writes; they are ioctls.  As far as I know, the kernel doesn't have any 
defined interface for zerocopy ioctls.

Furthermore, this approach _does_ use the mmap for data transfers.  I'm 
not sure what Christoph was referring to.

Alan Stern

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