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Sad: DMA limits

Forum: The Linux Kernel Hackers' Guide
Re: Question DMA buffer sizes
Keywords: DMA limit
Date: Sat, 26 Jul 1997 02:16:07 GMT
From: Albert Cahalan <acahalan at> <unknown>

> I want to allocate a DMA buffer larger than 128*1024.
> Is it possible to configure the kernel to allow bigger
> DMA buffer sizes? I've attempted to increase the PAGE_SIZE
> to 8192 but that crashes the system.

I hope that is not PC hardware! The ISA bus has a hard limit of 128 kB (16-bit DMA) or 64 kB (8-bit).

Even 128 kB is hard though, because memory fragmentation makes it unlikely that you can allocate a contiguous 128 kB chunk under the 16 MB ISA DMA limit (or elsewhere).