On Tue, Sep 28, 2004 at 12:08:31PM +0200, Thomas Petazzoni wrote: > My physical memory mapping is a bit special : I have 384 MB of > memory. The first 256MB are directly connected to the RM9000, while > the last 128MB are connected to the Marvell controller. _Only_ the > last 128MB are usable for DMA (especially for network traffic). For > the moment, Linux only takes care of the first 256MB, but I can change > it to take care of the complete physical memory space (384 MB). > > My problem is the allocation of skbuff. They are allocated using > alloc_skb() in net/core/skbuff.c, and uses the "normal" kmalloc() > allocator. kmalloc() will allocate memory somewhere in the physical > memory space : even if a I allow Linux to allocate memory between > 256MB and 384MB, I cannot be sure that it will use memory in this > space to allocate skbuff. If skbuff are not allocated in this space, > then I can't use DMA to transfer the buffers. > > As I understand the ZONE_DMA thing, it allows to tell Linux that a > physical memory region located between 0 and some value (16 MB on PCs > for old ISA cards compatibility) is the only area usable for DMA. How > could I declare my 256MB-384MB physical memory reagion to be the only > area usable for DMA ? How can I tell the skbuff functions to allocate > _only_ DMA-able memory ? ZONE_DMA has a system specific meaning. On a PCI system ISA could always be exist through a PCI-to-ISA bridge, so you can't just go and give it a system specific meaning. It's also needed for PCI devices with a less than 32-bit DMA limit; those exist in a rich variety. > Moreover, can I make assumptions on the > alignement of final data at the bottom of the network stack (my DMA > controller doesn't like the 2 byte-aligned things). Well, if you put packets on an aligned address you'll later take a bunch of missalignment exceptions which are going to severly impact networking performance ... > At the moment, I see only three solutions. The two first aren't not > very satisfying, the third might be a solution, but not perfect > neither (and not sure it would work). Change the configuration of the board to put the MV memory at the bottom. Leave ZONE_DMA what it used to be, < 16MB. Set the ZONE_NORMAL limit to 128MB. Anything above that is non-dmable will go into ZONE_HIGHMEM. See also CONFIG_LIMITED_DMA in 2.6. It works, it has little compatibility problems but it's a solution for platform that simply doesn't reflect the Linux hw architecture very much ... Ralf
