Re: [PATCH 03/10] x86: add initialization code for DMA-APIdebugging

From: Ingo Molnar
Date: Sun Nov 23 2008 - 06:36:23 EST



* Ingo Molnar <mingo@xxxxxxx> wrote:

>
> * Joerg Roedel <joro@xxxxxxxxxx> wrote:
>
> > On Fri, Nov 21, 2008 at 06:43:48PM +0100, Ingo Molnar wrote:
> > >
> > > * Joerg Roedel <joerg.roedel@xxxxxxx> wrote:
> > >
> > > > +static struct list_head dma_entry_hash[HASH_SIZE];
> > > > +
> > > > +/* A slab cache to allocate dma_map_entries fast */
> > > > +static struct kmem_cache *dma_entry_cache;
> > > > +
> > > > +/* lock to protect the data structures */
> > > > +static DEFINE_SPINLOCK(dma_lock);
> > >
> > > some more generic comments about the data structure: it's main purpose
> > > is to provide a mapping based on (dev,addr). There's little if any
> > > cross-entry interaction - same-address+same-dev DMA is checked.
> > >
> > > 1)
> > >
> > > the hash:
> > >
> > > + return (entry->dev_addr >> HASH_FN_SHIFT) & HASH_FN_MASK;
> > >
> > > should mix in entry->dev as well - that way we get not just per
> > > address but per device hash space separation as well.
> > >
> > > 2)
> > >
> > > HASH_FN_SHIFT is 1MB chunks right now - that's probably fine in
> > > practice albeit perhaps a bit too small. There's seldom any coherency
> > > between the physical addresses of DMA - we rarely have any real
> > > (performance-relevant) physical co-location of DMA addresses beyond 4K
> > > granularity. So using 1MB chunking here will discard a good deal of
> > > random low bits we should be hashing on.
> > >
> > > 3)
> > >
> > > And the most scalable locking would be per hash bucket locking - no
> > > global lock is needed. The bucket hash heads should probably be
> > > cacheline sized - so we'd get one lock per bucket.
> >
> > Hmm, I just had the idea of saving this data in struct device. How
> > about that? The locking should scale too and we can extend it
> > easier. For example it simplifys a per-device disable function for
> > the checking. Or another future feature might be leak tracing.
>
> that will help with spreading the hash across devices, but brings in
> lifetime issues: you must be absolutely sure all DMA has drained at
> the point a device is deinitialized.

Note that obviously proper DMA quiescence is a must-have during device
dinit anyway, but still, it's an extra complication to init/deinit the
hashes, etc.

Ingo
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