Dear Jonathan: 2011/11/1 Jonathan Corbet <corbet@lwn.net>:

On Tue, 1 Nov 2011 19:32:38 +0900 loody <miloody@gmail.com> wrote:

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My questions are below:

As are some answers :) That will be great appreciated :)

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1. Are all struct page 32-bit aligned?

Certainly, I would expect any structures found in the system memory map to be so aligned, yes.  If you've created an arbitrary one elsewhere I'm only 99% sure it would be.

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If so, where is this part of code for handleing struct page be 32-bits aligned?

Note that the size of struct page is carefully considered, and that they are packed as densely as possible, both externally and internally.  No lesser alignment would make sense. I found kernel stealing LSB of page_link to put additional information, such as sg_is_chain and sg_is_last.

That means the address of all pages have to be at least 4byte alignment. But I didn't see any additional attribute added at the end of struct or any precaution to take care this.

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2. if the page of scatterlist is located at high mem, how does dma_map_sg mapping the virtual address?

That depends on a vast array of things.  What are the addressing capabilities of the target device?  Is there an IOMMU involved?

No, my device has no iommu. and I found where kernel handle this part.

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3. in blk_rq_map_sg, it seems we only copy bvec->bv_page to sg->page_link    why don't we add dma_address in bvec and send the bio directly to the lower level driver?

That would be a bit of a mixing of the layers, if nothing else.

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4. if I plug a USB HD with 3 partitions, will there be 3 request_queues?

No, there is one request queue for the underlying device.

I study some block devices and it seems we use blk_init_queue for getting a request_queue, if we want block layer handle requests and coalesces adjacent requests. what happen and lower driver need to do if we don't use blk_init_queue to register a request_queue?

That's an interesting mix of questions.  If you have more in the future, you'll likely get more (and more clueful) responses if they're accompanied by the code you're working on.

Thanks,

jon

-- Regards,