linux-kernel - Re: [PATCH 03/10] x86: add initialization code for DMA-API debugging

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Message-ID: <20081121171004.GE1386@amd.com>
Date:	Fri, 21 Nov 2008 18:10:04 +0100
From:	Joerg Roedel <joerg.roedel@....com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	iommu@...ts.linux-foundation.org
Subject: Re: [PATCH 03/10] x86: add initialization code for DMA-API debugging

On Fri, Nov 21, 2008 at 05:56:28PM +0100, Ingo Molnar wrote:
> > +	return (entry->dev_addr >> HASH_FN_SHIFT) & HASH_FN_MASK;
> > +}
> > +
> > +static struct dma_debug_entry *dma_entry_alloc(void)
> > +{
> > +	gfp_t gfp = GFP_KERNEL | __GFP_ZERO;
> > +
> > +	if (in_atomic())
> > +		gfp |= GFP_ATOMIC;
> > +
> > +	return kmem_cache_alloc(dma_entry_cache, gfp);
> 
> hm. that slab allocation in the middle of DMA mapping ops makes me a 
> bit nervous. the DMA mapping ops are generally rather atomic.
> 
> and in_atomic() check there is a bug on !PREEMPT kernels, so it wont 
> fly.

I am not sure I understand this correctly. You say the check for
in_atomic() will break on !PREEMPT kernels?

> We dont have a gfp flag passed in as all the DMA mapping APIs really 
> expect all allocations having been done in advance already.

Hmm, I can change the code to pre-allocate a certain (configurable?)
number of these entries and disble the checking if we run out of it.

> Any chance to attach the debug entry to the iotlb entries themselves - 
> either during build time (for swiotlb) or during iommu init time (for 
> the hw iommu's) ?

Hm, I want to avoid that. This approach has the advantage that is
works independent of any dma_ops implementation. So it can be used to
find out if a DMA bug originates from the device driver or (in my case)
from the IOMMU driver.

Joerg

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