Re: [PATCH] mtd: rawnand: denali: add DT property to specify skipped bytes in OOB

[Miquel Raynal <miquel.raynal@bootlin.com>]

Hi Masahiro,

Masahiro Yamada <yamada.masahiro@socionext.com> wrote on Sat, 8 Sep
2018 01:10:25 +0900:

> Hi Boris,
> 
> 2018-09-07 23:53 GMT+09:00 Boris Brezillon <boris.brezillon@bootlin.com>:
> > On Fri, 7 Sep 2018 23:42:53 +0900
> > Masahiro Yamada <yamada.masahiro@socionext.com> wrote:
> >  
> >> Hi Boris,
> >>
> >> 2018-09-07 23:08 GMT+09:00 Boris Brezillon <boris.brezillon@bootlin.com>:  
> >> > Hi Masahiro,
> >> >
> >> > On Fri,  7 Sep 2018 19:56:23 +0900
> >> > Masahiro Yamada <yamada.masahiro@socionext.com> wrote:
> >> >  
> >> >> NAND devices need additional data area (OOB) for error correction,
> >> >> but it is also used for Bad Block Marker (BBM).  In many cases, the
> >> >> first byte in OOB is used for BBM, but the location actually depends
> >> >> on chip vendors.  The NAND controller should preserve the precious
> >> >> BBM to keep track of bad blocks.
> >> >>
> >> >> In Denali IP, the SPARE_AREA_SKIP_BYTES register is used to specify
> >> >> the number of bytes to skip from the start of OOB.  The ECC engine
> >> >> will automatically skip the specified number of bytes when it gets
> >> >> access to OOB area.
> >> >>
> >> >> The same value for SPARE_AREA_SKIP_BYTES should be used between
> >> >> firmware and the operating system if you intend to use the NAND
> >> >> device across the control hand-off.
> >> >>
> >> >> In fact, the current denali.c code expects firmware to have already
> >> >> set the SPARE_AREA_SKIP_BYTES register, then reads the value out.
> >> >>
> >> >> If no firmware (or bootloader) has initialized the controller, the
> >> >> register value is zero, which is the default after power-on-reset.
> >> >>
> >> >> In other words, the Linux driver cannot initialize the controller
> >> >> by itself.  You cannot support the reset control either because
> >> >> resetting the controller will get register values lost.
> >> >>
> >> >> This commit adds a way to specify it via DT.  If the property
> >> >> "denali,oob-skip-bytes" exists, the value will be set to the register.  
> >> >
> >> > Hm, do we really need to make this config customizable? I mean, either
> >> > you have a large-page NAND (page > 512 bytes) and the 2 first bytes
> >> > must be reserved for the BBM or you have a small-page NAND and the BBM
> >> > is at position 4 and 5. Are you sure people configure that differently?
> >> > Don't you always have SPARE_AREA_SKIP_BYTES set to 6 or 2?  
> >>
> >>
> >> As I said in the patch description,
> >> I need to use the same SPARE_AREA_SKIP_BYTES value
> >> across firmware, boot-loader, Linux, and whatever.
> >>
> >> I want to set the value to 8 for my platform
> >> because the on-chip boot ROM expects 8.
> >> I cannot change it since the boot ROM is hard-wired.
> >>
> >>
> >> The boot ROM skips 8 bytes in OOB
> >> when it loads images from the on-board NAND device.
> >>
> >> So, when I update the image from U-Boot or Linux,
> >> I need to make sure to set the register to 8.
> >>
> >> If I update the image with a different value,
> >> the Boot ROM fails to boot.
> >>
> >>
> >>
> >> When the system has booted from NAND,
> >> the register is already set to 8.  It works.
> >>
> >> However, when the system has booted from eMMC,
> >> the register is not initialized by anyone.
> >> I am searching for a way to set the register to 8
> >> in this case.
> >>
> >>
> >> The boot ROM in SOCFPGA might expect a different value,
> >> I am not sure.  
> >
> > Okay, then why not having a per-compatible value if it's related to the
> > BootROM? Unless the BootROM is part of the FPGA and can be
> > reprogrammed.  
> 
> FPGA is unrelated here.
> 
> Neither the boot ROM nor the Denali core is re-programmable.
> 
> 
> 
> I hesitate to associate the number of skipped bytes
> with the compatible string because it is not a parameter
> of the Denali IP.
> 
> 
> Rather, it is the matter of "how we use the OOB",
> so I want to leave room for customization like nand-ecc-strength etc.
> even if the boot ROM happens to expect a particular value.
> 
> 
> If you prefer a per-compatible value, I can do that,
> but I believe the NAND core and the boot ROM are orthogonal.
> 
> 
> 
> > I'd really prefer not having a generic property that
> > allows you to put anything you want.  
> 
> 

While I agree that the number of skipped bytes is not a parameter of
the Denali IP, I also fear letting the opportunity to the user to use
random values in the SPARE_AREA_SKIP_BYTES registers (and have to
support them). I would also prefer a per-compatible value which is not
a perfect solution neither.


Thanks,
Miquèl