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Using blk_make_request() kAPI for REQ_TYPE_BLOCK_PC commands

Posted by dwata 
Using blk_make_request() kAPI for REQ_TYPE_BLOCK_PC commands
March 25, 2011 04:44AM

I need clarification about blk_make_request() usage. (Kernel 2.6.38).

1. [My understanding/Facts]

i) accroding to the source file comment,
blk_make_request() is "is the parallel of generic_make_request for
BLOCK_PC type commands".

ii) blk_make_request() calls:-
-> blk_rq_append_bio()
-> (under some condtion) ll_back_merge_fn()

In ll_back_merge_fn(),
there is a if-condition like below:-

if (unlikely(blk_pc_request(req)))
max_sectors = queue_max_hw_sectors(q);
max_sectors = queue_max_sectors(q)

iii) blk_make_request() calls
blk_get_request(), and calls blk_rq_append_bio() (see above).
req->cmd_type is _not_ set in blk_make_request().
the if-condition (in l_back_merge_fn() (see above)) executes
"max_sectors = queue_max_sectors(q)".

So, if I call blk_make_request(),
max_sectors will be _always_ "queue_max_sectors(q)",
rather than queue_max_hw_sectors(q).

This is my understanding.

2. [Question/Clarification Point]

Q1) Is there any "proper" way to use blk_make_request(),
let the code use queue_max_hw_sectors(q) as max_sectors?

Q2) If blk_make_request() is not expecting to be called for
should I add a new function like,

struct request *rq = blk_get_request(q, bio_data_dir(bio), gfp_mask);
rq->cmd_type = REQ_TYPE_BLOCK_PC;

for_each_bio(bio) {

return rq;

And, use this blk_make_pc_request or something
If I really need to have a execution path,
-> blk_rq_append_bio()
-> ll_back_merge_fn()
-> max_sectors = queue_max_hw_sectors(q);

Is this a valid way?

It is very apprecaited if anybody can give me a clue.

Thanks very much.

D. Watanabe
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