On Friday, May 30, 2014 12:55:06 pm Attilio Rao wrote: > On Fri, May 30, 2014 at 6:44 PM, John Baldwin <j...@freebsd.org> wrote: > > On Friday, May 30, 2014 11:51:38 am Attilio Rao wrote: > >> On Fri, May 30, 2014 at 5:47 PM, John Baldwin <j...@freebsd.org> wrote: > >> > On Friday, May 30, 2014 11:39:24 am Attilio Rao wrote: > >> >> On Fri, May 30, 2014 at 5:03 PM, John Baldwin <j...@freebsd.org> wrote: > >> >> > On Friday, May 30, 2014 10:54:06 am Attilio Rao wrote: > >> >> >> On Tue, May 27, 2014 at 11:31 PM, Scott Long <sco...@freebsd.org> wrote: > >> >> >> > Author: scottl > >> >> >> > Date: Tue May 27 21:31:11 2014 > >> >> >> > New Revision: 266775 > >> >> >> > URL: http://svnweb.freebsd.org/changeset/base/266775 > >> >> >> > > >> >> >> > Log: > >> >> >> > Eliminate the fake contig_dmamap and replace it with a new flag, > >> >> >> > BUS_DMA_KMEM_ALLOC. They serve the same purpose, but using the flag > >> >> >> > means that the map can be NULL again, which in turn enables significant > >> >> >> > optimizations for the common case of no bouncing. > >> >> >> > >> >> >> While I think this is in general a good idea, unfortunately our > >> >> >> drivers do so many dumb things when freeing DMA allocated buffers that > >> >> >> having a NULL map is going to cause some "turbolence" and make such > >> >> >> bugs more visible. > >> >> >> An example is with ATA, where I think this fix is needed: > >> >> >> http://www.freebsd.org/~attilio/dmamem_free-ata.patch > >> >> >> > >> >> >> Otherwise, what can happen with bounce buffers, is that the allocated > >> >> >> memory via contig malloc was not going to be freed anytime. > >> >> >> > >> >> >> I tried to look around and I found questionable (read broken) code in > >> >> >> basically every driver which allocates DMA buffers, so I really don't > >> >> >> feel I want to fix the majority of our drivers. I just think such > >> >> >> paths are not excercised enough to be seen in practice often or the > >> >> >> bugs just get unnoticed. > >> >> > > >> >> > Eh, many maps for static allocations were already NULL and have been for a > >> >> > long time. This is nothign new. Plus, the diff you posted has a bug > >> >> > regardless of explicitly destroying a map created by bus_dmamem_alloc(). > >> >> > >> >> Did you notice that I *removed* the destroy not *added*? > >> > > >> > Yes, my point was that that bug in the original code you are fixing was there > >> > regardless of Scott's change. > >> > >> And when I did say something different? > >> I don't understand what's the point of your messages, besides showing > >> that you didn't read correctly my patch. > > > > I read yours correctly but worded mine poorly. My point is that Scott's > > change does not introduce anything new. We've had NULL maps for static > > allocations for many, many years. It's only been recently that we've > > had more maps not be NULL for this. However, even if you discounted > > the whole NULL vs non-NULL maps thing, the driver in question that you > > are fixing was broken regardless. That is, due to the extra > > bus_dmamap_destroy() the driver was broken regardless of whether the map > > was NULL or non-NULL. > > To be honest, pre-266775 the kernel would actually panic for this > specific driver, because we were going to free memory that was never > allocated (by having a valid mapping but an invalid dma memory > pointer).
pre-239354 bus_dma would have used a NULL map just as it does now. And even some allocations during that window could still use a NULL map. The idea of a NULL map is not a new concept. Most maps from bus_dmamem_alloc() have been NULL for most of bus_dma's existence. > That was prompted to look at the dma_alloc_*() bits of drivers. > We need to make a real sweep at drivers on these bits. I did a start: http://p4web.freebsd.org/@@1194266?ac=10 -- John Baldwin _______________________________________________ svn-src-all@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/svn-src-all To unsubscribe, send any mail to "svn-src-all-unsubscr...@freebsd.org"
