Re: RFC: turn scatterlist into a linked list, eliminate bio_vec

From: Adam J. Richter (
Date: Mon Jun 24 2002 - 06:42:45 EST

>From: "David S. Miller" <>
>Date: Sun, 23 Jun 2002 23:24:16 -0700 (PDT)

>> /* Let's assume the new pci_map_sg will free unused scatterlists. */
>> while (sg != NULL && count--) {
>> sg->driver_priv = mempool_alloc(q->sgpriv_pool, GFP_KERNEL);
>> sg->driver_priv_dma =
>> pci_map_single(req->dma_map_dev, sg->driver_priv, len,
>> if (sg_dma->dma_add_priv == 0);
>> failed = fail_value;
>> }
>> }

>Driver descriptors are not supposed to be done using pci_map_*()
>and friends. You are supposed to use consistent DMA memory for
>this purpose. Please read DMA-mapping.txt a few more times Adam :-)

        Sorry if I was not clear enough about the purpose of of
the new scatterlist->driver_priv field. It is a "streaming" data
structure to use the terminology of DMA-maping.txt (i.e., one that
would typically only be allocated for a few microseconds as an IO is
built up and sent). Its purpose is to hold the hardware-specific
gather-scatter segment descriptor, which typically look something like this:

                struct foo_corporation_scsi_controller_sg_element {
                        u64 data_addr;
                        u64 next_addr;
                        u16 data_len;
                        u16 reserved;
                        u32 various_flags;
        I probably should not have mentioned adding sg->driver_priv{,_dma}
in my proposal, because it is largely independent. The reason that
it is related is that it should allow the "mid-layer" device driver code
(scsi.c, usb.c, etc.) to do all of the streaming DMA mapping for
typical DMA-based device drivers.

        Come to think of it, my use of pci_map_single is incorrect
after all, because the driver has not yet filled in that data structure
at that point. Since the data structures are being allocated from a
single contiguous block that spans a couple of pages that is being used
only for this purpose, perhaps I would be fastest to pci_alloc_consistent
the whole memory pool for those little descriptors at initialization time
and then change that loop to do the following.

                sg->driver_priv = mempool_alloc(q->sgpriv_pol, GFP_KERNEL);
                sg->driver_priv_dma = (sg->driver_priv - q->driver_priv_start)+

>Also, this while loop never terminates :-)

        Oops! Sorry. The bottom of that loop needs

                sg = sg->next;

Adam J. Richter __ ______________ 575 Oroville Road \ / Milpitas, California 95035
+1 408 309-6081 | g g d r a s i l United States of America
                         "Free Software For The Rest Of Us."
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