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

From: Shuah Khan <shuah.khan@hp.com>
To: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: fujita.tomonori@lab.ntt.co.jp,
	LKML <linux-kernel@vger.kernel.org>,
	akpm@linux-foundation.org, paul.gortmaker@windriver.com,
	bhelgaas@google.com, amwang@redhat.com, shuahkhan@gmail.com
Subject: Re: [PATCH RFC] swiotlb: Remove SWIOTLB overflow buffer support
Date: Tue, 10 Jul 2012 17:13:58 -0600	[thread overview]
Message-ID: <1341962038.2502.54.camel@lorien2> (raw)
In-Reply-To: <1341961616.2502.52.camel@lorien2>

On Tue, 2012-07-10 at 17:06 -0600, Shuah Khan wrote:
> 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.

Correction. CONFIG_ISA_DMA_API is made configurable on x86_64 it appears
back in 2011.

-- Shuah