Date: Thu, 12 Jul 2012 08:26:05 -0400 From: John Baldwin <jhb@freebsd.org> To: freebsd-hackers@freebsd.org Cc: Alan Cox <alc@freebsd.org>, Warner Losh <imp@freebsd.org> Subject: Re: contigmalloc() breaking Xorg Message-ID: <201207120826.05577.jhb@freebsd.org> In-Reply-To: <20120708110516.GA38312@server.rulingia.com> References: <20120703111753.GB72292@server.rulingia.com> <20120708110516.GA38312@server.rulingia.com>
next in thread | previous in thread | raw e-mail | index | archive | help
[ Adding alc@ for VM stuff, Warner for arm/mips bus dma brokenness ] On Sunday, July 08, 2012 7:05:16 am Peter Jeremy wrote: > On 2012-Jul-03 21:17:53 +1000, Peter Jeremy <peter@server.rulingia.com> wrote: > >I have a reasonably recent 8-stable/amd64 system (r237444) with a "ATI > >Radeon HD 2400 Pro", xorg-server-1.10.6,1 and xf86-video-ati-6.14.3_1 > >8GB RAM and ZFS. I'm seeing fairly consistent problems with Xorg > ... > >How difficult would it be to modify bus_dmamem_alloc() [at least on > >x86] to handle multi-segment allocations? > > I think I've managed to create an amd64 bus_dmamem_alloc() that allows > page-sized allocations as long as no boundary condition is specified > and no more than page-sized alignment is required (porting it to other > architectures would be trivial). I've given it a quick whirl inside a > VBox and no smoke came out but I'd appreciate someone with a better > understanding of bus_dma(9) and vm/vm_contig.c giving > http://www.rulingia.com/bugs/patch-wiredmalloc a once-over. Note that > this patch is against 8.x but there's only a trivial difference to head. > > BTW, the comment in busdma_machdep.c:bus_dmamem_alloc() > * XXX Use Contigmalloc until it is merged into this facility > * and handles multi-seg allocations. Nobody is doing > * multi-seg allocations yet though. > * XXX Certain AGP hardware does. > does not appear to be accurate. Apart from drm, quite a few drivers > call bus_dma_tag_create(9) with multiple segments and also call > bus_dmamem_alloc(9) [though I haven't verified that the calls share > the same bus_dma_tag, so I can't be absolutely certain]. I do think that all tags currently used with bus_dmamem_alloc() only use a single segment. It's a bit of an unfortunate part of the bus_dmamem API that the size of the allocate is determined by the tag (the tag should be used for determining the features and constraints of a DMA engine, not really the amount of memory to allocate). However, rather add a wiredmalloc(), I think you should just have bus_dmamem_alloc() call kmem_alloc_attr() directly in this case. One of the things I've been meaning to add to bus_dma is a way to allocate other memory types (e.g. WC memory), and in that case it would be best to just call kmem_alloc_attr() directly instead. > BTW(2): Whilst studying busdma_machdep.c for arm and mips, I've > noticed they appear to potentially allocate substantial kernel stack > under some conditions as several bus_dma(9) functions include: > bus_dma_segment_t dm_segments[dmat->nsegments]; > What prevents this overflowing the kernel stack? That does seem dubious. x86 stores the array in the tag instead. -- John Baldwin
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?201207120826.05577.jhb>