NDCBx registers in PXA3XX MTD driver

[Romain Bornet]

Hi Eric

In fact I didn't pay attention to the note at the beginning of the
chapter and to the fact that NDCB1 and 2 are specified read-only!

Thanks for the clarification and the quick reply!

Have a nice day.
    Romain

On Thu, Feb 18, 2010 at 1:39 PM, Eric Miao <eric.y.miao at gmail.com> wrote:
> On Thu, Feb 18, 2010 at 8:06 PM, Romain Bornet <bornet.romain at gmail.com> wrote:
>> Hi folks,
>>
>> I'm rather new in the ARM / Linux world and currently working on a
>> PXA168-based board.
>>
>> I'm trying to understand the implementation of the pxa3xx_nand.c
>> driver and came across 3 lines of code I cannot understand.
>>
>> The 3 lines in question are the one where the NDCB0/1/2 registers are
>> written in write_cmd()
>> ---
>> nand_writel(info, NDCB0, info->ndcb0);
>> nand_writel(info, NDCB0, info->ndcb1);
>> nand_writel(info, NDCB0, info->ndcb2);
>> ---
>>
>> see here for context...
>> http://lxr.linux.no/#linux+v2.6.32/drivers/mtd/nand/pxa3xx_nand.c#L459
>>
>> Why are all 3 values (ndcb0, ndcb1, ndcb2) written to the same
>> register NDCB0 whereas the PXA3xx and PXA168 have different offsets
>> (0x48, 0x4C and 0x50) for these 3 registers as documented in
>> PXA3xx_DM_Vol_II in chapter "3.8.10 NAND Controller Command Buffers
>> (NDCBx)" (available from Marvell @
>> http://www.marvell.com/products/processors/applications/pxa_3xx/PXA3xx_Developers_Manual.zip
>> )
>>
>> I first thought about a possible hardware Errata but couldn't find any
>> hint in this direction.
>>
>> I would have expected
>>
>> nand_writel(info, NDCB0, info->ndcb0);
>> nand_writel(info, NDCB1, info->ndcb1);
>> nand_writel(info, NDCB2, info->ndcb2);
>>
>
> If you read the documentation a little bit more carefully, you'll know why.
>
> Basically, the write is to NDCB0 _only_ and in turn writing to 0, 1, 2
> respectively, reading of NDCB1 and NDCB2 is however, doable, which
> will show you what you've written.
>