linux-kernel - Re: [PATCH v3 2/4] mtd: rawnand: Add Macronix MX25F0A NAND controller

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Message-ID: <20190527144250.71908bd9@xps13>
Date:   Mon, 27 May 2019 14:42:50 +0200
From:   Miquel Raynal <miquel.raynal@...tlin.com>
To:     masonccyang@...c.com.tw
Cc:     bbrezillon@...nel.org, broonie@...nel.org,
        christophe.kerello@...com, computersforpeace@...il.com,
        devicetree@...r.kernel.org, dwmw2@...radead.org,
        geert@...ux-m68k.org, juliensu@...c.com.tw, lee.jones@...aro.org,
        liang.yang@...ogic.com, linux-kernel@...r.kernel.org,
        linux-mtd@...ts.infradead.org, linux-spi@...r.kernel.org,
        marcel.ziswiler@...adex.com, marek.vasut@...il.com,
        mark.rutland@....com, paul.burton@...s.com, richard@....at,
        robh+dt@...nel.org, stefan@...er.ch, zhengxunli@...c.com.tw
Subject: Re: [PATCH v3 2/4] mtd: rawnand: Add Macronix MX25F0A NAND
 controller

Hi Mason,

masonccyang@...c.com.tw wrote on Thu, 23 May 2019 16:58:02 +0800:

> Hi Miquel,
> 
> > >   
> > > > > +
> > > > > +static void mxic_nand_select_chip(struct nand_chip *chip, int   
> chipnr) 
> > > > 
> > > > _select_target() is preferred now   
> > > 
> > > Do you mean I implement mxic_nand_select_target() to control #CS ?
> > > 
> > > If so, I need to call mxic_nand_select_target( ) to control #CS ON
> > > and then #CS OFF in _exec_op() due to nand_select_target()<in   
> nand_base,c>  
> > > is still calling chip->legacy.select_chip ?  
> > 
> > You must forget about the ->select_chip() callback. Now it should be
> > handled directly from the controller driver. Please have a look at the
> > commit pointed against the marvell_nand.c driver.  
> 
> I have no Marvell NFC datasheet and have one question.
> 
> In marvell_nand.c, there is no xxx_deselect_target() or 
> something like that doing #CS OFF.
> marvell_nfc_select_target() seems always to make one of chip or die
> #CS keep low.
> 
> Is it right ?

Yes, AFAIR there is no "de-assert" mechanism in this controller.

> 
> How to make all #CS keep high for NAND to enter 
> low-power standby mode if driver don't use "legacy.select_chip()" ?

See commit 02b4a52604a4 ("mtd: rawnand: Make ->select_chip() optional
when ->exec_op() is implemented") which states:

        "When [->select_chip() is] not implemented, the core is assuming
	the CS line is automatically asserted/deasserted by the driver
	->exec_op() implementation."

Of course, the above is right only when the controller driver supports
the ->exec_op() interface. 

So if you think it is not too time consuming and worth the trouble to
assert/deassert the CS at each operation, you may do it in your driver.


Thanks,
Miquèl