Re: Is usb_buffer_alloc a generic URB buffer allocator ? (was Split Bulk Transfers)

On Sat, 19 May 2007, jidong xiao wrote:

> The length of any entry should be greater than a page?I think only if each
> buffer's size is divisible by page size, could it start at a page boundary
> and end at a page boundary.

Let's use the word "buffer" to refer to the region of memory described by
the entire scatter-gather list.  Let's use the word "sub-buffer" to refer
to that portion of the buffer which is described by a single scatterlist
entry.  Note that each entry corresponds to a group of pages (or a single
page), only part of which might be contained in the sub-buffer.

If the sub-buffer begins at the start of the group of pages and ends at
the end of the group of pages then its length will indeed be a multiple of
the page size.  Each sub-buffer (except possibly the first one) will begin
at the start of the page group.  And each sub-buffer (except possibly the 
last one) will end at the end of the page group.

Another way to describe this is as follows: For each scatterlist entry
after the first, we should always have .offset = 0.  For each scatterlist
entry before the last, we should always have (.offset + .length) is a
multiple of the page size.

Alan Stern


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