linux-kernel - Re: [PATCH v2] Add support for usbfs zerocopy.

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Date:	Mon, 11 Jan 2016 12:08:26 +0100
From:	Oliver Neukum <oneukum@...e.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Lingzhu Xiang <lingzhu.xiang@...l.utoronto.ca>,
	"Steinar H. Gunderson" <sesse@...gle.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	USB list <linux-usb@...r.kernel.org>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] Add support for usbfs zerocopy.

On Thu, 2016-01-07 at 10:40 -0500, Alan Stern wrote:
> > I understand there are some requirements on the allocation such that
> > large blocks are not always available. But what is the proper way to
> > determine the upper limit of the size such that the user can avoid
> > generating warnings like this? (Also, the application really wants
> to
> > be able to allocate large buffers, maybe tune swiotlb=?.)
> 
> It's debatable whether this should have generated a warning.  Why 
> doesn't dma_alloc_coherent() simply fail silently?

I suspect many drivers to be unable to deal well with a failure.
Having this report makes "my device doesn't work" easier to solve
as a bug report.

Hence it seems to me that a driver which can handle a failure
with a good fallback should indicate this with a flag to the
VM layer.

	Regards
		Oliver