linux-kernel - Re: [PATCH v4] drivers/block/mtip32xx: Adding new driver mtip32xx

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Date:	Tue, 13 Sep 2011 09:46:07 -0700
From:	Eric Seppanen <eds@...ic.net>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	"Sam Bradshaw (sbradshaw)" <sbradshaw@...ron.com>,
	jaxboe@...ionio.com, alan@...rguk.ukuu.org.uk,
	linux-ide@...r.kernel.org, linux-kernel@...r.kernel.org,
	Jeff Garzik <jgarzik@...ox.com>, jmoyer@...hat.com,
	"Asai Thambi Samymuthu Pattrayasamy (asamymuthupa) [CONTRACTOR]" 
	<asamymuthupa@...ron.com>
Subject: Re: [PATCH v4] drivers/block/mtip32xx: Adding new driver mtip32xx

On Tue, Sep 13, 2011 at 5:49 AM, Christoph Hellwig <hch@...radead.org> wrote:
> Btw, there is another _huge_ issue with the driver, and that is the
> lack of any internal queueing.  Remember the make_request interface is
> an extremly thin layer (or rather the lack of it) below the filesystem.
>
> So for example if eh_active is non-zero you return -EBUSY to the
> filesystems.  That's an error code it a) doesn't recognize and b)
> couldn't handle even if it did.  Similarly mtip_hw_get_scatterlist
> simply blocks if no tag is currently available instead of queueing
> it up.

Just out of curiosity, why is blocking on no-tag-available a bad
thing?  How is it any different than the blocking that will occur when
a request queue is full?  When the hardware queue depth is bigger than
that of a request queue, what extra benefit does queuing give?
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