linux-kernel - Re: [PATCH 0/7] sg_ring: a ring of scatterlist arrays

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Message-ID: <4781C8B5.6050207@gmail.com>
Date:	Mon, 07 Jan 2008 15:37:41 +0900
From:	Tejun Heo <htejun@...il.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	James Bottomley <James.Bottomley@...senpartnership.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org,
	linux-ide@...r.kernel.org
Subject: Re: [PATCH 0/7] sg_ring: a ring of scatterlist arrays

Rusty Russell wrote:
> On Monday 07 January 2008 16:01:40 Tejun Heo wrote:
>>> But we hit the same problems:
>>>
>>> 1) sg_chain loses information.  The clever chain packaging makes reading
>>> easy, but manipulation is severely limited.  You can append to your own
>>> chains by padding, but not someone elses.  This works for SCSI, but what
>>> about the rest of us?  And don't even think of joining mapped chains: it
>>> will almost work.
>> You can append by allocating one more element on the chain to be
>> appended and moving the last element of the first chain to it while
>> using the last element for chaining.
> 
> Hi Tejun,
> 
>    Nice try!  Even ignoring the ugliness of undoing such an operation if the 
> caller doesn't expect you to mangle their chains, consider a one-element sg 
> array. :(

Heh heh, that can be dealt with by skipping the first chain if the first
chain is empty after chaining.  Please take a look at
ata_sg_setup_extra() in the following.

http://git.kernel.org/?p=linux/kernel/git/tj/libata-dev.git;a=blob;f=drivers/ata/libata-core.c;h=32dde5bbc75ed53e89ac17040da2cd0621a37161;hb=c8847e473a4a2844244784226eb362be10d52ce9

That said, yeah, it's seriously ugly.  Restoring the original sg is ugly
too.  I definitely agree that we need some improvements here.

-- 
tejun
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