Re: [PATCH] mtd: nand: gpmi: add proper raw access support

From: Huang Shijie <shijie8@gmail.com>
To: Boris BREZILLON <boris.brezillon@free-electrons.com>
Cc: Brian Norris <computersforpeace@gmail.com>,
	Huang Shijie <shijie.huang@intel.com>,
	Mike Voytovich <mvoytovich@paypal.com>,
	linux-kernel@vger.kernel.org, Huang Shijie <b32955@freescale.com>,
	linux-mtd@lists.infradead.org, Roy Lee <roylee@paypal.com>,
	David Woodhouse <dwmw2@infradead.org>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] mtd: nand: gpmi: add proper raw access support
Date: Wed, 17 Sep 2014 23:26:11 +0800	[thread overview]
Message-ID: <20140917152609.GA2111@localhost.localdomain> (raw)
In-Reply-To: <20140915221210.5d9871fb@bbrezillon>

On Mon, Sep 15, 2014 at 10:12:10PM +0200, Boris BREZILLON wrote:
> On Mon, 15 Sep 2014 22:43:02 +0800
> Huang Shijie <shijie8@gmail.com> wrote:
> 
> > On Sat, Sep 13, 2014 at 10:38:41AM -0700, Brian Norris wrote:
> > > On Sat, Sep 13, 2014 at 11:36:24PM +0800, Huang Shijie wrote:
> > > > On Fri, Sep 12, 2014 at 02:30:50PM +0200, Boris BREZILLON wrote:
> > > > > This test validates what's returned by ecc_strength file in sysfs
> > > > > (which in turn is specified by the NAND controller when initializing
> > > > > the NAND chip).
> > > > > 
> > > > > Doing this should not imply knowing the ECC algorithm in use in the
> > > > > NAND controller or the layout used to store data on NAND.
> > > > the difficulty is that the ECC parity area can be not byte aligned.
> > > 
> > > Is there a problem with just rounding up to the nearest byte alignment
> > > and ignoring the few bits that are wasted?
> > 
> > I feel a little confused with the two hooks.
> > 
> > does the ecc->write_page_raw need to write the ECC parity data?
> 
> Depending on the oob_required argument, it might be allowed to
> overwrite the ECC bytes even if this implies breaking page reliability
> (which is exactly what's expected).
> 
> When using raw write with with oob write option the writer should take
> care of regenerating ECC bytes (which you said was impossible in GPMI
> case) or copying them from a previous raw read.
Thanks for the explanation.

If we do not write the OOB, should we write the ECC bytes?
The hooks should comment clearly about how to implement them :(

> 
> Here is a real example of what one could test with raw write + oob:
> 1) read a page in raw mode
> 2) flip some bits in the generated ECC bytes (or what you references as
>    parity data) (this case can actually happen in real life)
> 3) write the modified page in raw mode
> 4) read back the same page in normal and check that ECC correction still
>    works as expected
the nandbiterr test mode does the test as above.
But i think the multi-writes to the same page should occur only for the
SLC nand. 

I will read your new patch set carefully in this weekend.

thanks
Huang Shijie