How to combine multiple struct BIOs into a single struct request?

Question

I'm working on Linux kernel version 2.6.39.1, and am developing a block device driver. In this regard, I want to combine multiple struct bios into a single struct request, which is then added to the request_queue for processing by the device driver, namely -- scsi_request_fn().

I tried using the ->bi_next field of struct bio to link multiple struct bios that I have composed, thereby creating a linked list of struct bios. When I call submit_bio() to submit a bio to the block device layer for I/O, this BUG_ON() is triggered because the code expects bio->bi_next to be NULL.

Is there a way to link several struct bios into a single struct request before sending it to lower layers for servicing?

Answer 1

I'm not sure how to string multiple struct bio together, but you might want to take a look at the "task collector" implementation in libsas and the aic94xx driver for an alternate approach. There isn't much documentation, but the libsas documentation describes it as

Some hardware (e.g. aic94xx) has the capability to DMA more than one task at a time (interrupt) from host memory. Task Collector Mode is an optional feature for HAs which support this in their hardware. (Again, it is completely optional even if your hardware supports it.)

In Task Collector Mode, the SAS Layer would do natural coalescing of tasks and at the appropriate moment it would call your driver to DMA more than one task in a single HA interrupt. DMBS may want to use this by insmod/modprobe setting the lldd_max_execute_num to something greater than 1.

Effectively, this lets the block layer (a.k.a. BIO) remain unchanged, but multiple requests are accumulated at the driver layer and submitted together.

Answer 2

Thanks for the reply, @ctuffli. I've decided to use a structure similar to the one described here. Basically, I allocate a struct packet_data which would contain pointers to all struct bios that should be merged to form one single struct bio (and later on, one single struct request). In addition, I store some driver related information as well in this struct packet_data. Next, I allocate a new struct bio (lets call it "merged_bio"), copy all the pages from the list of original BIOs and then make the merged_bio->bi_private point to the struct packet_data. This last hack would allow me to keep track of the list of original BIOs, and also call bio_endio() to end I/O on all individual BIOs once the merged_bio has been successfully transferred.

Not sure if this is the smartest way to do this, but it does what I intended! :^)