svn commit: r279828 - head/sys/dev/cadence

[Ian Lepore]

On Tue, 2015-03-10 at 13:33 -0600, Scott Long wrote:
> > On Mar 9, 2015, at 4:39 PM, Ian Lepore <ian at FreeBSD.org> wrote:
> > 
> > Author: ian
> > Date: Mon Mar  9 22:39:58 2015
> > New Revision: 279828
> > URL: https://svnweb.freebsd.org/changeset/base/279828
> > 
> > Log:
> >  Use the new ifnet API.  Also, allocate bus_dma_maps as needed instead of
> >  pre-allocating them all at start-up.  Also fix a bug in cgem_stop(); before,
> >  it wasn't properly unloading dma maps due to a cut-and-paste error.
> > 
> 
> It’s generally a very bad idea to create and destroy maps for every transaction.  If the map
> results in being non-NULL, then you’re allocating and freeing memory on every transaction,
> and possibly allocating and freeing large chunks of memory for bounce buffers.  Is there
> a reason why you’ve made this change?

Maps are always non-NULL on arm.  Bounce pages are managed in pools and
the initial minimum allocation happens when the first mapping is
created, then extended as needed to keep a minimum of two bounce pages
per map using that pool.  Since mbufs never actually bounce on arm the
pool just sits there at minimum allocation size, wasting a bit of memory
but not costing any cycles on subsequent mappings, unless there are many
maps created at once, in which case each new mapping created adds a
couple more pages to the pool (which will still never get used).

So creating 1024 maps all at once results in a bounce pool with 2048
pages in it, none of which will ever be used in the case of mbufs
(because mbufs get to violate the rules about the buffer being aligned
to a cacheline boundary, which is the primary cause of bouncing on arm).

-- Ian