On Fri, Jul 29, 2016 at 3:14 PM, François <fser@code-libre.org> wrote:
Hi there,
I've been reading LDD3's chapter 16 on block devices a few times, and have made toys block layers module. Now I've been looking at the up-to-date examples provided by martinez javier[1], but still there's a fundamental concept I fail to understand.
Considering only the RM_FULL and RM_SIMPLE cases, a request queue is created, bound with a lock, and associated with a request function.
In the simple case, that function "sbull_request" processes each request from the request queue, and delegates the work to "sbull_transfer", which basically performs some arithmetic and does the actual data copy. This function is given a sector, a number of sectors, a pointer to a buffer, and a read or write parameter extracted from the request using blk_rq_pos(), blk_rq_cur_sectors(), req->buffer and rq_data_dir() respectively.
On the other hand, the same mechanism is used, but a different function is associated: "sbull_full_request". That function also extracts requests and delegates the actual work to "sbull_xfer_request" which iterates on the request's bio, calls "sbull_xfer_bio" which itself iterates on the bio's segments and finally, calls the same "sbull_transfer" function.
What I fail to understand is: how (with the same initialization) the behaviour of the module using those two somehow different mechanism is equivalent.
I don't see req->buffer. Which version you are using?
One has to understand the full complexity of the underlying data structure (requests having bio, having segments) while the other only reads the containing structure (the struct request) and do the same job, without iterations.
Bonus point, to give some context: I'm writing an asynchronous block-layer which has to split requests into custom subrequest. I'm wondering which approach (between those two) I should pickup.
If this is a memory backed block driver, then perhaps you can handle multiple requests[?]. I don't think you need to actually break up the same request into multiple requests.
Thanks for reading so far, and for any hints :)
[1] https://github.com/martinezjavier/ldd3/blob/master/sbull/sbull.c -- François
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