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RE: Socket buffer allocation outside DMA-able memory

To: "Zhan, Rongkai" <>, "ashley jones" <>, "art" <>, <>
Subject: RE: Socket buffer allocation outside DMA-able memory
From: "Zhan, Rongkai" <>
Date: Tue, 6 Jun 2006 16:02:55 +0200
Original-recipient: rfc822;
Thread-index: AcaJZjbxp1IvfUOrSwyzJ2H7FF15GwABt95QAAEMDCA=
Thread-topic: Socket buffer allocation outside DMA-able memory
After having a look at the latest 2.6.17-rc6 codes, __dev_alloc_skb is defined 
like this:

 *      __dev_alloc_skb - allocate an skbuff for sending
 *      @length: length to allocate
 *      @gfp_mask: get_free_pages mask, passed to alloc_skb
 *      Allocate a new &sk_buff and assign it a usage count of one. The
 *      buffer has unspecified headroom built in. Users should allocate
 *      the headroom they think they need without accounting for the
 *      built in space. The built in space is used for optimisations.
 *      %NULL is returned in there is no free memory.
static inline struct sk_buff *__dev_alloc_skb(unsigned int length,
                                              gfp_t gfp_mask)
        struct sk_buff *skb = alloc_skb(length + NET_SKB_PAD, gfp_mask);
        if (likely(skb))
                skb_reserve(skb, NET_SKB_PAD);
        return skb;
extern struct sk_buff *__dev_alloc_skb(unsigned int length, int gfp_mask);

Therefore, you also can consider to implement your machine-specific 
__dev_alloc_skb() function, and force the skb is allocated from your low 32MB 
memory zone.

Best Regards,
Mark. Zhan
[] On Behalf Of Zhan, Rongkai
Sent: Tuesday, June 06, 2006 9:34 PM
To: ashley jones; art;
Subject: RE: Socket buffer allocation outside DMA-able memory


Maybe you can enable ISA bus. And then add your low 32MB memory into ZONE_DMA, 
while the high 32MB memory into ZONE_NORMAL.
In the case, you are required to redefine MAX_DMA_ADDRESS to (PAGE_OFFSET + 
0x00200000) in asm-mips/dma.h

Just a noise.

Best Regards,
Mark. Zhan
[] On Behalf Of ashley jones
Sent: Tuesday, June 06, 2006 8:38 PM
To: art;
Subject: Re: Socket buffer allocation outside DMA-able memory

       I guess your 25 bit dma address field will be word alligned, so ur dma 
engine will be able to index up to 64 MB( 25+2 = 27 bits).
        If that is not the case then you can use one of the foll. work around,
  * dont give whole 64 MB to linux, give only Lower 32 MB.
  * Give only upper 32 MB to linux, and give memory to ur dma engine from lower 
32 MB, and once you recv any data you copy to skb and submit to linux. ( 
ofcourse your performance will get hit in this case.)

art <> wrote:
Hello all!
I work with ADM5120 chip. it has embedded switch.
Switch descriptor has 25-bit dma addres field - so addressible only
My system has 64Mb memory. But I have to set 32Mb to make it work!
I thought that setting DMA mask can help. So in
/arch/mips/adm5120/setup.c i make:

static struct platform_device adm5120hcd_device = {
.name = "adm5120-hcd",
.id = -1,
.dev = {
.dma_mask = &hcd_dmamask,
.coherent_dma_mask = 0x01ffffff,
.num_resources = ARRAY_SIZE(adm5120_hcd_resources),
.resource = adm5120_hcd_resources,
But It is wrong, because dev_alloc_skb dont know to which device it
allocates buffer!

How to tell kernel allocate skbuffers in less then 32Mb addrspace whet
system has 64Mb?

Best regards,

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