Date:	Mon, 16 Jul 2012 10:47:21 -0600
From:	Shuah Khan <shuah.khan@...com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:	fujita.tomonori@....ntt.co.jp, LKML <linux-kernel@...r.kernel.org>,
	akpm@...ux-foundation.org, paul.gortmaker@...driver.com,
	bhelgaas@...gle.com, amwang@...hat.com, shuahkhan@...il.com
Subject: Re: [PATCH RFC] swiotlb: Disable swiotlb overflow support when
 CONFIG_ISA is enabled

On Mon, 2012-07-16 at 12:01 -0400, Konrad Rzeszutek Wilk wrote:
> On Mon, Jul 16, 2012 at 09:48:20AM -0600, Shuah Khan wrote:
> > On Mon, 2012-07-16 at 10:45 -0400, Konrad Rzeszutek Wilk wrote:
> > > On Thu, Jul 12, 2012 at 10:17:50AM -0600, Shuah Khan wrote:
> > > > Disable iotlb overflow support when CONFIG_ISA is enabled. This is the
> > > > first step towards removing overflow support, to be consistent with other
> > > > iommu implementations and return DMA_ERROR_CODE. This disabling step is
> > > > for finding drivers that don't call dma_mapping_error to check for errors
> > > > returned by the mapping interface. Once drivers are fixed overflow support
> > > > can be removed.
> > > 
> > > I think I explained myself poorly. We don't want to break old drivers.
> > > 
> > > If the old drivers expect this behavior (while they should be updated
> > > to check the DMA), we need to retain this behavior.
> > > 
> > > So I think the patch should be done the other way:
> > > 
> > > Disable IOTLB overflow when CONFIG_ISA is disabled.
> > 
> > That makes lot of sense :) I will redo the patch. It might be good to a
> > tunable to control enable/disable behavior. Even though support for this
> > new tunable "swiotlb_overflow" projected to be short lived, it will be
> > lot less painful to disable/enable as needed. Thought and comments?
> 
> Tunnable have a habit of becoming permanant and we really don't want that.
> The right solution is to drop the overflow - as you have rightly
> pointed out in the first patch. But the road is blocked by these old
> drivers that are making it hard.
> 
> I would say if you really really want a patch in for 3.6 before the
> final mega-patch of drivers-that-suck-and-need-to-be-updated-here-is
> -the-big-patch-that-does-it, then just add a WARN mentioning your
> name and mine (and LKML) saying to report what you see. That way
> we can at least get feedback from the field.
> 
> And for good measure include in the Documentation/*deprecate-schedule
> that the overflow functionality is going away when all the drivers that
> depend on it have been fixed up.

Will work on it and get the patch with deprecation schedule document out
soon.

> 
> > 
> > > 
> > > But we also need to check whether there are other drivers
> > > that don't properly check the DMA address. And if so, add them
> > > to this list of must have enabled b/c you might be using this
> > > driver.
> > 
> > Sound good. Again having a tunable would help in this case as well.
> > > 
> > > The first goal is to figure out which of the drivers aren't doing this
> > > properly. This should be possible by just grepping for 'dma_map' and 
> > > seeing which ones don't do the 'dma_check' right after.
> > 
> > I started some research into this and will continue and provide an
> > update.
> 
> Thank you! It is pretty griddy work so I really appreciate you taking
> the time to do this. If are at the LinuxCon this year the beers (or coffee
> if that is your preference) are on me for tackling on this cleanup.

Thanks. Unfortunately I won't be at this year's LinuxCon, maybe I get to
take you up on your offer of coffee at one of the future ones.

-- Shuah

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