linux-kernel - Re: can block drivers get non-fs requests?

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Message-ID: <20080814114209.GB20055@kernel.dk>
Date:	Thu, 14 Aug 2008 13:42:09 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Pierre Ossman <drzeus-list@...eus.cx>
Cc:	Alex Dubov <oakad@...oo.com>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: can block drivers get non-fs requests?

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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