Re: [PATCH v10 3/5] mtd: nand: vf610_nfc: add device tree bindings

[Stefan Agner <stefan@agner.ch>]

On 2015-08-26 08:39, Boris Brezillon wrote:
> Hi Bill,
> 
> On Wed, 26 Aug 2015 11:26:36 -0400
> Bill Pringlemeir <bpringle@sympatico.ca> wrote:
> 
>> On 25 Aug 2015, computersforpeace@gmail.com wrote:
>>
>> > Sorry, I realized a potential issue here.
>>
>> > On Mon, Aug 03, 2015 at 11:27:28AM +0200, Stefan Agner wrote:
>> >> Signed-off-by: Bill Pringlemeir <bpringlemeir@nbsps.com>
>> >> Acked-by: Shawn Guo <shawnguo@kernel.org>
>> >> Reviewed-by: Brian Norris <computersforpeace@gmail.com>
>> >> Signed-off-by: Stefan Agner <stefan@agner.ch>
>> >> ---
>> >> .../devicetree/bindings/mtd/vf610-nfc.txt | 45 ++++++++++++++++++++++
>> >> 1 file changed, 45 insertions(+) create mode 100644
>> >> Documentation/devicetree/bindings/mtd/vf610-nfc.txt
>>
>> >> diff --git a/Documentation/devicetree/bindings/mtd/vf610-nfc.txt
>> >> b/Documentation/devicetree/bindings/mtd/vf610-nfc.txt
>> >> new file mode 100644
>> >> index 0000000..cae5f25
>> >> --- /dev/null
>> >> +++ b/Documentation/devicetree/bindings/mtd/vf610-nfc.txt
>> >>>> -0,0 +1,45 @@
>> >> +- nand-bus-width: see nand.txt
>> >> +- nand-ecc-mode: see nand.txt
>> >> +- nand-on-flash-bbt: see nand.txt
>>
>> > Stumbling across the "multi-CS" questions on the driver reminds me: it
>> > typically makes sense to define new NAND bindings using separate NAND
>> > *controller* and *flash* device nodes. The above 3 properties, at
>> > least, would apply on a per-flash basis, not per-controller
>> > typically. See sunxi-nand, for instance:
>>
>> > http://lxr.free-electrons.com/source/Documentation/devicetree/bindings/mtd/sunxi-nand.txt
>>
>> > brcmnand had a similar pattern:
>>
>> > https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/devicetree/bindings/mtd/brcm,brcmnand.txt
>>
>> > (Perhaps it's time we standardized this a little more formally...)
>>
>> These would apply per chip, but the controller has to be configured to
>> support each and every one.  Every time an operation was performed, we
>> would have to check the chip type and reconfigure the controller.
>> Currently, the driver does not support this and it would add a lot of
>> overhead in some cases unless a register cache was used.
>>
>> Is the flexibility of using a system with combined 8/16bit devices
>> really worth all the overhead?  Isn't it sort of brain dead hardware not
>> to make all of the chips similar?  Why would everyone have to pay for
>> such a crazy setup?
>>
>> To separate it would at least be a lie versus the code in the current
>> form.  As well, there are only a few SOC which support multiple chip
>> selects.  The 'multi-CS' register bits of this controller varies between
>> PowerPC, 68K/Coldfire and ARM platforms.

The DT can be a lie versus the code. The DT should reflect how the
hardware is wired, afaik, if we take shortcuts in the driver code, that
is fine. If we don't support a certain configuration right now (e.g.
second NAND chip), the driver can just return an appropriate error code.
>>
>> I looked briefly at the brcmnand.c  and it seems that it is not
>> supporting different ECC per chip even though the nodes are broken out
>> this way.  In fact, if some raw functions are called, I think it will
>> put it in ECC mode even if it wasn't before?  Well, I agree that this
>> would be good generically, I think it puts a lot of effort in the
>> drivers for not so much payoff?
> 
> Hm, the sunxi driver supports it, and it does not add such a big
> overhead...
> The only thing you have to do is cache a bunch of register values
> per-chip and restore/apply them when the chip is selected
> (in your ->select_chip() implementation).
> 
> Anyway, even if the suggested DT representation is a lie in regards to
> your implementation, it's actually pretty accurate from an hardware
> POV, and this is exactly what DT is supposed to represent.

I agree with both of you. I don't see much value implementing multi-NAND
chip support, especially with different configurations, at the moment. I
am not aware of any hardware making use of that now.

I will update the driver to parse a NAND sub node and get the ECC
properties from the per flash configuration. However, I won't add chip
select or multi-NAND support right now...

Any objection?

--
Stefan