bus_dmamem_alloc strangeness

[Harti Brandt]

On Thu, 17 Feb 2005, Gerald Heinig wrote:

GH>Hi hackers,
GH>
GH>I've come across weird behaviour in bus_dmamem_alloc() whilst trying to
GH>allocate small memory blocks (less than PAGE_SIZE) which have to be
GH>aligned to PAGE_SIZE.
GH>My segment size is 2048, my maximum number of segments is 1 (it MUST be
GH>contiguous), my max. total segment size is also 2048 and my alignment
GH>constraint is 4k (PAGE_SIZE). However, the DMA memory I'm getting from
GH>bus_dmamem_alloc() is NOT aligned to 4k.
GH>
GH>The relevant code in sys/i386/i386/busdma_machdep.c is:
GH>
GH>=============================================================
GH>
GH>	if ((dmat->maxsize <= PAGE_SIZE) &&
GH>	    dmat->lowaddr >= ptoa((vm_paddr_t)Maxmem)) {
GH>		*vaddr = malloc(dmat->maxsize, M_DEVBUF, mflags);
GH>	} else {
GH>		/*
GH>		 * XXX Use Contigmalloc until it is merged into this facility
GH>		 *     and handles multi-seg allocations.  Nobody is doing
GH>		 *     multi-seg allocations yet though.
GH>		 * XXX Certain AGP hardware does.
GH>		 */
GH>		*vaddr = contigmalloc(dmat->maxsize, M_DEVBUF, mflags,
GH>		    0ul, dmat->lowaddr, dmat->alignment? dmat->alignment :
GH>1ul,
GH>		    dmat->boundary);
GH>	}
GH>==============================================================
GH>
GH>My lowaddr is BUS_SPACE_MAXADDR and I'm allocating 2k blocks which have to be
GH>4k-aligned, which would imply the first if branch.
GH>
GH>Surely the code should adhere to the alignment restrictions and not simply
GH>allocate memory without checking, or am I missing something here?

I discovered this problem when I wrote my ATM drivers years ago. My 
workaround is to make sure that size >= alignment for all memory blocks by 
just allocating larger blocks. This seems just to work up to now.

harti