On Fri, Jul 29, 2016 at 3:14 PM, François <[email protected]> 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 > > _______________________________________________ > Kernelnewbies mailing list > [email protected] > https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies -- ---P.K.S _______________________________________________ Kernelnewbies mailing list [email protected] https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
