linux-kernel - Re: New Mitsumi legacy CD-ROM driver

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Date:	Tue, 8 May 2007 12:35:24 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Rene Herman <rene.herman@...il.com>
Cc:	Andrew Morton <akpm@...l.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: New Mitsumi legacy CD-ROM driver

On Tue, May 08 2007, Rene Herman wrote:
> Good day.
> 
> This is (the start of) a new driver for the legacy Mitsumi CD-ROM (LU-00xS 
> and FX-001x) drives that used to be supported by drivers/cdrom/mcd(x).c, a 
> driver that hasn't worked for a long time.
> 
> It's being tested on my LU-005S and is being submitted to -mm in the hope 
> of maybe finding a few more testers and also in the hope of provoking a 
> review. I'm co-authoring this with Pekka Enberg but certainly for me it's 
> the first ever contact with anything having anything to do with block.
> 
> A few specific points/questions:
> 
> * This driver's core transfer function is doing:
> 
>   rq_for_each_bio(bio, req)
>       bio_for_each_segment(bvec, bio, segno) {
>           buffer = bvec_kmap_irq(bvec, &flags);
>           ioread8_rep(ioaddr, buffer, bvec->bv_len);
>           bvec_kunmap_irq(buffer, &flags);
>       }
> 			
> I've been slightly worried about struct bio having a bi_sector field. If 
> we're not guaranteed that bio[n].bi_sector == bio[n-1].bi_sector + 1 during 
> the rq_for_each_bio() loop we can't read from I/O into bio's directly but 
> will need to use a temporary buffer and transfer into the bio's later. Is 
> this guaranteed?

bio[n - 1].bi_sector + bio_sectors(bio[n - 1]) == bio[n].bi_sector

IOW, the bio's in the chain on the request are contig in the lba space.

> * This driver purports to support highmem. Given the number of machines out 
> there that have both a Mitsumi legacy CD-ROM drive and enough physical 
> memory installed to want highmem support this may come across as being 
> overly... optimistic but is there much point to not supporting it? I 
> believe we do always need the full rq_for_each_bio/bio_for_each_segment 
> loop meaning that only the bvec_k{,un}map_irq could go, right?

Yeah, I think it's pointless to support highmem. And as long as you
don't call blk_queue_bounce_limit() to actively enable highmem, you are
not going to receive a bio/request with highmem pages. The block layer
wil bounce any highmem pages before it reaches you. So for a driver like
this, I would recommend just forgetting the mapping aspect.

I don't have time to review your driver right now, but I will applaud
your effort to write a maintenable mitsumi driver! Basically all the old
cdrom drivers are utter crap, so they are destined for removal.

-- 
Jens Axboe

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