Messages in this thread | | | Date | Thu, 14 Aug 2008 13:42:09 +0200 | From | Jens Axboe <> | Subject | Re: can block drivers get non-fs requests? |
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On Thu, Aug 14 2008, Pierre Ossman wrote: > Hi Jens, > > There is one part of the block layer I'm having trouble completely > understanding (well there are loads really, but right now there's one in > particular :)): > > Do block drivers need to watch out for requests that aren't "normal" > reads and writes? The MMC block driver has a funky prepare function > that checks for FS and PC requests and discards anything else. I > believe this is wrong in two regards:
You should check in the driver whether the request type is supported. So if you can only handle fs requests, you should do a check for !blk_fs_request() after elv_next_request() and call end_(de)queued_request() appropriately.
> a) PC requests seem to be prepared SCSI cmds, and the MMC driver just > expects normal transfer requests. Therefore, such requests should not > be accepted by this driver.
Yes, that is correct. A PC request is essentially a SCSI command sent directly.
> b) The block layer won't throw weird stuff at us unless explicitly told > so. I.e. a normal block queue will only produce normal FS requests.
Normal IO activity will only produce fs requests. However, if someone uses eg SG_IO on your block device, you will be sent pc requests. There are no checks for this, it is left to the driver.
-- Jens Axboe
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