On Tuesday, August 31, 2010 1:33:48 pm Pyun YongHyeon wrote:
> Author: yongari
> Date: Tue Aug 31 17:33:48 2010
> New Revision: 212061
> URL: http://svn.freebsd.org/changeset/base/212061
> Split common parent DMA tag into ring DMA tag and TX/RX mbuf DMA
> tag. All controllers that are not BCM5755 or higher have 4GB
> boundary DMA bug. Previously bge(4) used 32bit DMA address to
> workaround the bug(r199670). However this caused the use of bounce
> buffers such that it resulted in poor performance for systems which
> have more than 4GB memory. Because bus_dma(9) honors boundary
> restriction requirement of DMA tag for dynamic buffers, having a
> separate TX/RX mbuf DMA tag will greatly reduce the possibility of
> using bounce buffers. For DMA buffers allocated with
> bus_dmamem_alloc(9), now bge(4) explicitly checks whether the
> requested memory region crossed the boundary or not.
> With this change, only the DMA buffer that crossed the boundary
> will use 32bit DMA address. Other DMA buffers are not affected as
> separate DMA tag is created for each DMA buffer.
> Even if 32bit DMA address space is used for a buffer, the chance to
> use bounce buffer is still very low as the size of buffer is small.
> This change should eliminate most usage of bounce buffers on
> systems that have more than 4GB memory.
> More correct fix would be teaching bus_dma(9) to honor boundary
> restriction for buffers created with bus_dmamem_alloc(9) but it
> seems that is not easy.
> While I'm here cleanup bge_dma_map_addr() and remove unnecessary
> member variables in bge_dmamap_arg structure.
Keep in mind the PAE case where you cannot effectively specify a 4GB
boundary. I used a 2GB boundary for twa(4) in the PAE case to deal
with the boundary issue. Probably though, bus_dma should just always
enforce a 4GB boundary, at least on x86.