Re: Getting/Forcing Greater than 4KB Buffer Allocations

On 7/17/07, Pyun YongHyeon <pyunyh_at_gmail.com> wrote:
> On Tue, Jul 17, 2007 at 04:54:31PM -0700, David Christensen wrote:
>  > I'm investigating a problem with my bce driver which occurs when I ask
>  > for a jumbo
>  > mbuf cluster (through m_cljget()).  When I map the memory for DMA I
>  > normally
>  > get 3 memory segments (4KB + 4KB + 1KB) on my system, but on another
>  > user's
>  > system he's seeing 2 memory segments (8KB + 1KB).  Is there a
>  > configuration
>  > option that allows this or some other tuning variable involved?  The
>  > system is a
>  > Xeon dual-core processor and has 8GB of RAM, running an AMD64 version of
>  > the kernel.
>  >
>
> I've briefly looked over bus_dma usage on bce(4). It seems that you
> told bus_dma the the dma map could be made up of BCE_MAX_SEGMENTS
> segments, where a dma segment could be MJUM9BYTES bytes. If you want
> just two segments you may have to use 2 instead of BCE_MAX_SEGMENTS.
> If the hardware can support up to BCE_MAX_SEGMENTS dma segments on Rx
> descriptors you should be prepared to handle that number of dma
> segments too(e.g. You don't know how may dma segments would be
> returned by bus_dma, you just know the upper bound as you specified
> in bus_dma_tag_create()).
> If the hardware can handle just up to 4KB for a dma segment you
> should tell bus_dma the restriction of the dma segment.
> If you have to get a single dma segment that covers MJUM9BYTES bytes
> due to the limitation of the hardware you may have to use local
> allocator.

I have a patch to make jumbo frames contiguous that will go into HEAD
after 7 branches.

 -Kip