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Date:   Tue, 15 Oct 2019 09:56:37 +0200
From:   Miquel Raynal <miquel.raynal@bootlin.com>
To:     masonccyang@mxic.com.tw
Cc:     "Boris Brezillon" <boris.brezillon@collabora.com>,
        bbrezillon@kernel.org, computersforpeace@gmail.com,
        dwmw2@infradead.org, frieder.schrempf@kontron.de,
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        linux-kernel@vger.kernel.org, linux-mtd@lists.infradead.org,
        marcel.ziswiler@toradex.com, marek.vasut@gmail.com, richard@nod.at,
        tglx@linutronix.de, vigneshr@ti.com
Subject: Re: [PATCH RFC 3/3] mtd: rawnand: Add support Macronix power down
 mode
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Hi Mason,

masonccyang@mxic.com.tw wrote on Tue, 15 Oct 2019 10:33:29 +0800:

> Hi Boris,
> 
>  
> > > > > +   nand_select_target(chip, 0);   
> > > > 
> > > > On several NAND controllers there is no way to act on the CS line
> > > > without actually writing bytes to the NAND chip. So basically this
> > > > is very likely to not work.   
> > > 
> > > any other way to make it work ? GPIO ?
> > > or just have some comments description here.
> > > i.e,.
> > > 
> > > /* The NAND chip will exit the deep power down mode with #CS toggling,   
> 
> > >  * please refer to datasheet for the timing requirement of tCRDP and   
> tRDP.
> > >  */
> > >   
> > 
> > Good luck with that. As Miquel said, on most NAND controllers
> > select_target() is a dummy operation that just assigns nand_chip->target
> > to the specified value but doesn't assert the CS line. You could send a
> > dummy command here, like a READ_ID, but I guess you need CS to be
> > asserted for at least 20ns before asserting any other signals (CLE/ALE)
> > which might be an issue.  
> 
> okay, got it.
> But if possible, I think adding CS line control in nand_select_target()
> is a simple and generic way for MTD and all raw NAND controllers.

The problem is not that we do not want to; the problem is that
controllers are not capable of doing it reliably if no byte is sent
over the NAND bus.


Thanks,
Miquèl