[RFC][patch] NAND partial page read functionality

[Alexey Korolev]

Hamish,

> Thanks for your patch Alexey.
> 
> On Thu, Apr 24, 2008 at 11:25:01AM +0100, Alexey Korolev wrote:
> > Oh I'm glad to hear that this patch is needful. In fact I did
> > not drop it. We did some more investigations about integration subpage
> >  read in background. We tried to remove some drawback caused by
> > invalidation of prestored page in do_read_ops function. (We have found
> > that on large read requests nand subpage read causes ~10% of read performance degradation. I
> > wanted to remove but changes made code ugly and saved only 5%)
> 
> Do you know why this is? If you are reading a whole page then the
> partial read code is equivalent to the read full page code as far as I
> can tell.
Yes. Sometimes it is better to read one page and place it into a cache
than reading part of it several times. 
The weak places of original implementation 
1.  we read whole OOB each time. 
2. Many subpage requests occures sequentally we often have to reread 256bytes
fragment. 

As result sequential read of large contineous chunks is 10% worse.
I did not found any other drawbacks of this implementation. 


> 
> > > Also Alexey said:
> > > > Setting NAND_PART_READ option in board driver will enable this
> > > > feature.
> > > 
> > > which is not true, as nand_base.c masks out chip-options (like this)
> > > inherited from the board driver. It must be set for the chip in
> > > nand_ids.c instead.
> > It works fine,  you need to setup this option after nand_scan. 
> > Using it in NAND_IDS is bad idea because some people may want disable it you can indirectly identify if chip
> > supports subpage read from NAND capabilities records.y 
> 
> Oh, I see. I think it is unusual to modify the chip->options after
> calling nand_scan? I am using plat_nand.c and it doesn't have any way to
> do this.
> 
> Maybe NAND_PART_READ should be excluded from NAND_CHIPOPTIONS_MSK. But
> you still have the case where the chip can't do it even if the board
> wants to. Maybe the chip should declare it's capable and the board
> should be able to turn it off if required.
> 
> Mind you, other parts of nand_base use the RNDOUT function already so I
> don't think there is any case where you *wouldn't* enable partial reads.
> 
> > So if there is an interest in subpage read let resume the topic. In next couple days I
> > will send updated patch with subpage read functionality. 
> 
> Thanks. Did you find any actual bugs with the previous patch, or is it
> safe for me to use it?

Yes. It is safe. We passed excessive tests before I sent it
out. 

Thanks,
Alexey