linux-kernel - Re: [PATCH RFC] swiotlb: Remove SWIOTLB overflow buffer support

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Date:	Tue, 10 Jul 2012 17:06:56 -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
Subject: Re: [PATCH RFC] swiotlb: Remove SWIOTLB overflow buffer support

On Tue, 2012-07-10 at 13:32 -0400, Konrad Rzeszutek Wilk wrote:
> On Tue, Jul 10, 2012 at 10:55:07AM -0600, Shuah Khan wrote:
> > On Mon, 2012-07-09 at 16:25 -0400, Konrad Rzeszutek Wilk wrote:
> > > On Fri, Jul 06, 2012 at 05:06:12PM -0600, Shuah Khan wrote:
> > > > Remove SWIOTLB overflow buffer support and return DMA_ERROR_CODE
> > > > (a value of zero) to make it consistent with iommu implementation
> > > > on Intel, AMD, and swiotlb-xen.
> > > 
> > > While this is a good forward step and this needs to be done eventually,
> > > you should first send out patches for the drivers that don't check
> > > for the DMA_ERROR_CODE when doing mapping. In other words for the
> > > drivers that map but don't call dma_mapping_error to check.
> > > 
> > > When that is fixed and *all the drivers that don't call dma_mapping_error
> > > are fixed, then this patch makes sense.
> > 
> > The challenge will be catching all the drivers and have confidence that
> > all of them are covered. I will start looking into this to get a feel
> > for how many drivers needs fixing.
> 
> I don't know if all is needed. Some of them might be dead or not used
> at all anymore - who knows? This treasure hunt would give a good idea
> of which ones are not using the PCI/DMA API right at least.
> 
> If it is say, CONFIG_ISA enabled, well, we could #ifdef the overflow
> buffer in the swiotlb with that option. And then work through fixing
> up those drivers - except that finding folks with that driver to
> see if it works .. yuck.. I do have some few ISA cards and some boxes
> with ISA slots :-)

Looking at the history CONFIG_ISA was disabled back in 2002 and
CONFIG_ISA_DMA_API was disabled in 2004. Sounds like it will be fun
finding drivers that could fail. 

Would it make sense to make io_tlb_overflow a tunable and disable by
default instead of ifdef? Currently it is always enabled, disabling it
by default is another way to find problems?

Good to know you have a few ISA cards. :)

-- Shuah


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