Date:	Wed, 09 Mar 2011 19:24:55 +0200
From:	Maxim Levitsky <maximlevitsky@...il.com>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
Cc:	akpm@...ux-foundation.org, James.Bottomley@...senPartnership.com,
	linux-kernel@...r.kernel.org, oakad@...oo.com
Subject: Re: [PATCH 1/4] scatterlist: new helper functions

On Mon, 2011-03-07 at 04:20 +0200, Maxim Levitsky wrote:
> On Mon, 2011-03-07 at 06:49 +0900, FUJITA Tomonori wrote:
> > On Sun, 06 Mar 2011 17:14:30 +0200
> > Maxim Levitsky <maximlevitsky@...il.com> wrote:
> > 
> > > On Sun, 2011-03-06 at 16:29 +0900, FUJITA Tomonori wrote:
> > > > On Fri,  4 Mar 2011 06:16:50 +0200
> > > > Maxim Levitsky <maximlevitsky@...il.com> wrote:
> > > > 
> > > > > While developing memstick driver for legacy memsticks
> > > > > I found the need in few helpers that I think should be
> > > > > in common scatterlist library
> > > > > 
> > > > > The functions that were added:
> > > > > 
> > > > > * sg_nents/sg_total_len - iterate over scatterlist to figure
> > > > > out total length of memory it covers / number of entries.
> > > > 
> > > > You should invent a data structure per I/O request, something like
> > > > msb_request structure. Then you can store nents and total_len in
> > > > that.
> > > > 
> > > > That's what block subsystems and drivers do. I took a look at your
> > > > driver but I can't see why your driver can't do the same.
> > > I also need to break the request into small grained chunks.
> > > If I invent such structure, I will end up writing these helpers for it.
> > > 
> > > The I have this lifetime of a request:
> > > 
> > > I get arbitary sized request from block layer (I can of course control
> > > maximum size/number of segments in it, etc).
> > > 
> > > I break it into eraseblock sized chunks, and for each I translate the
> > > the LBA, into flash address.
> > > 
> > > Then I break it into flash page sized requests (512 bytes), and yet its
> > > better not to assume that such requests always contained in one sg
> > > entry.
> > > 
> > > Worse than that, I have to pass an sg list that spans always one 512
> > > page to lowlevel driver, because thats how Alex defined the interface.
> > 
> > This restriction is due to hardware specification or the software
> > design (e.g. memstick layer)? If it is due to the latter, why can't
> > you fix that?
> 
> Yes.
> I already tried addressing some shortcomings of memstick layer, no no, I
> don't want to deal with its author, Alex Dubov again.
> I think this code tries to be too clever/complex for the range of
> devices/speeds it supports, but I rather leave it as is.
> 
> 
> To be honest, the code in question is for >5 year old memstick standard
> cards, thats hardly anybody uses.
> It works, it is more or less simple, its not performance bound, its
> testd, and thus I want to keep it as is _for_ now.
> 
> 
> Why I break sg lists into chunks?
> Because unlike vast majority of block devices, I need to do FTL in the
> driver, thus its easier to work on eraseblock boundary.
> Also unlike anything else, you can't just read/write a sector from a
> memorystick (especially the legacy one), you have to perform full dance
> of commands.
> 
> Not to mention error handling (like if you failed to write to block, you
> must try to choose another one, etc...)
> 
> (Of course writes follow same rules as raw nand flash, thats is writes
> only clear bits, and you can erase a eraseblock only).
> 
> 
> 
> > 
> > Why can't the block layer split requests for you? It's better to let
> > the block layer handle that.
> You mean tell it not to give me more that one eraseblock to handle?
> Could you explain that a bit more?
> 
> 
> Anyway, could we merge the code?
> I would happy to improve it later, but currently merge window is very
> close, and the code is more or less agreed upon everyone, and written
> more that 1/2 of year ago.
> 
> Andrew Morton, could you help me with this, Please?
Ping (I really worry about this).

> 
> 
> Best regards,
> 	Maxim Levitsky
> 
> > 
> > 
> > > Folks, really what the status of this, when to expect it to be merged?
> > > 
> > > If you think some of helper functions don't belong to scatterlist.c,
> > > just tell me to move them back to ms_block.c.
> > > 
> > > Andrew, please note again that richoh lowlevel driver doesn't need any
> > > helper functions, its patch is standalone and thus should be merged
> > > regardless.
> > 
> > I think that we need to make the design of the driver easily
> > understandable to kernel developers and maintainable by them. I don't
> > think that this is 'standalone or not' issue.
> > 
> > Adding a doc about why the driver is designed in such odd way would be
> > helpful. But I still think that we could design the driver in a better
> > way.
> 
> 


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