Date: Thu, 17 Feb 2005 18:38:40 +0100 From: Gerald Heinig <gheinig@syskonnect.de> To: hackers@freebsd.org Subject: bus_dmamem_alloc strangeness Message-ID: <4214D6A0.9010803@syskonnect.de>
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Hi hackers, I've come across weird behaviour in bus_dmamem_alloc() whilst trying to allocate small memory blocks (less than PAGE_SIZE) which have to be aligned to PAGE_SIZE. My segment size is 2048, my maximum number of segments is 1 (it MUST be contiguous), my max. total segment size is also 2048 and my alignment constraint is 4k (PAGE_SIZE). However, the DMA memory I'm getting from bus_dmamem_alloc() is NOT aligned to 4k. The relevant code in sys/i386/i386/busdma_machdep.c is: ============================================================= if ((dmat->maxsize <= PAGE_SIZE) && dmat->lowaddr >= ptoa((vm_paddr_t)Maxmem)) { *vaddr = malloc(dmat->maxsize, M_DEVBUF, mflags); } else { /* * 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. */ *vaddr = contigmalloc(dmat->maxsize, M_DEVBUF, mflags, 0ul, dmat->lowaddr, dmat->alignment? dmat->alignment : 1ul, dmat->boundary); } ============================================================== My lowaddr is BUS_SPACE_MAXADDR and I'm allocating 2k blocks which have to be 4k-aligned, which would imply the first if branch. Surely the code should adhere to the alignment restrictions and not simply allocate memory without checking, or am I missing something here? Cheers, Gerald
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