linux-kernel - Re: Smartmedia/xd card support

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Date:	Sun, 20 Apr 2008 18:33:11 -0700 (PDT)
From:	Alex Dubov <oakad@...oo.com>
To:	Linux Kernel list <linux-kernel@...r.kernel.org>
Subject: Re: Smartmedia/xd card support - request for comments

> >
> > I wonder if mtd shouldn't modified to handler more clever controller :
> > I saw some raw nand controller where you can program a batch of 
> > command/read/write/ecc checking (that work on a internal RAM of the 
> > controller (and may be use DMA)) and all you have to do is to wait the 
> > controller do the job.
> > These controllers are very hard to integrate in current nand layer 
> > because it is very low level controller oriented.
> 
> The obvious solution is to have seperate io scheduling from io
> completion.  Requires an asynchronous interface to read/write and some
> sort of request queue.
> 
> Not quite as obvious is the question of whether to add all this to mtd
> or to replace mtd with block devices, which already have it.
> 

I was thinking about it myself and it appears to be the only solution outright.

The interesting, but apparently hard way to do so is to define a couple of
REQ_TYPE_SPECIAL mtd-specific commands and use the existing block device queue
to manipulate them. This can even open the possibility of "static" request
translation: mtd_blkdev can pop the block device request, put it through media
specific translator and then push a necessary set of mtd operations back into
the same queue, delegating the processing to raw mtd device.

The simpler approach I'm following with my xd_card_blk is to define a
simplistic, block device like api. First, the translator is initialized with
the request range (offset + length) and backend is notified to start
processing. The backend then pulls flash requests (READ, WRITE, COPY, ERASE)
from the translator, possibly putting them through additional, protocol/host
specific translation layer.

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