Re: [PATCH 02/13] IB/core: allow passing mapping an offset into the SG in ib_map_mr_sg

[Sagi Grimberg <sagig@dev.mellanox.co.il>]

On 29/02/2016 13:56, Christoph Hellwig wrote:
> On Mon, Feb 29, 2016 at 01:35:44PM +0200, Sagi Grimberg wrote:
>>> But for lager SG entries we need it to calculate the correct
>>> base address.
>>
>> I'm not sure if this is true either. Can you explain why?
>
> Assume we get a SG entry that is exactly 2 pages (8k) long.  But we
> also have an offset of 6k into it, so we need to skip into the
> second page to make the following work:
>
>>
>> The Memory region mapping is described by:
>> 1. page vector: [addr0, addr1, addr2,...]
>> 2. iova: the first byte offset
>> 3. length: the total byte count of the mr
>> 4. page_size: size of each page in the page vector
>>
>> This means that the HCA assumes that each address in
>> the page vector has the size of page_size, also the region
>> can start at some offset (iova - addr0), and it has some length.
>
> Exactly.
>
> For the above case we don't need the page at sg_dma_address(), though.
> We need the one after it, so we need to make sure the page address
> is calculated for the second page in the SG entry.
>
> If we add sg_offset to dma_addr is in my page this means we get the
> right page address from this line:
>
> 		u64 page_addr = dma_addr & page_mask;
>
> without that's we'd get the address of the first page, which doesn't
> actually contain any data we want to map.

Yea, makes sense...

I get it now! Thanks!