[U-Boot] [PATCH 1/4] spl: nand: simple: replace readb() with chip specific read_buf()

[Vladimir Zapolskiy]

Hello Albert,

On 16.07.2015 15:43, Albert ARIBAUD wrote:
> Hello Vladimir,
> 
> On Thu, 16 Jul 2015 14:31:26 +0300, Vladimir Zapolskiy <vz at mleia.com>
> wrote:
>> Hello Albert,
>>
>> On 16.07.2015 11:02, Albert ARIBAUD wrote:
>>> Hello Vladimir,
>>>
>>> On Thu, 16 Jul 2015 02:33:45 +0300, Vladimir Zapolskiy <vz at mleia.com>
>>> wrote:
>>>> Some NAND controllers define custom functions to read data out,
>>>> respect this in order to correctly support bad block handling in
>>>> simple SPL NAND framework.
>>>>
>>>> NAND controller specific read_buf() is used even to read 1 byte in
>>>> case of connected 8-bit NAND device, it turns out that read_byte()
>>>> may become outdated.
>>>>
>>>> Signed-off-by: Vladimir Zapolskiy <vz at mleia.com>
>>>> ---
>>>>  drivers/mtd/nand/nand_spl_simple.c | 7 +++++--
>>>>  1 file changed, 5 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/drivers/mtd/nand/nand_spl_simple.c b/drivers/mtd/nand/nand_spl_simple.c
>>>> index 700ca32..e69f662 100644
>>>> --- a/drivers/mtd/nand/nand_spl_simple.c
>>>> +++ b/drivers/mtd/nand/nand_spl_simple.c
>>>> @@ -115,6 +115,7 @@ static int nand_command(int block, int page, uint32_t offs,
>>>>  static int nand_is_bad_block(int block)
>>>>  {
>>>>  	struct nand_chip *this = mtd.priv;
>>>> +	u_char bb_data[2];
>>>>  
>>>>  	nand_command(block, 0, CONFIG_SYS_NAND_BAD_BLOCK_POS,
>>>>  		NAND_CMD_READOOB);
>>>> @@ -123,10 +124,12 @@ static int nand_is_bad_block(int block)
>>>>  	 * Read one byte (or two if it's a 16 bit chip).
>>>>  	 */
>>>>  	if (this->options & NAND_BUSWIDTH_16) {
>>>> -		if (readw(this->IO_ADDR_R) != 0xffff)
>>>> +		this->read_buf(&mtd, bb_data, 2);
>>>> +		if (bb_data[0] != 0xff || bb_data[1] != 0xff)
>>>>  			return 1;
>>>>  	} else {
>>>> -		if (readb(this->IO_ADDR_R) != 0xff)
>>>> +		this->read_buf(&mtd, bb_data, 1);
>>>> +		if (bb_data[0] != 0xff)
>>>>  			return 1;
>>>>  	}
>>>>  
>>>> -- 
>>>> 2.1.4
>>>>
>>>
>>> The way you describe this patch, it looks like a bug that should have
>>> bitten way more boards than lpc32xx. Did you have a look at some other
>>> boards to see why this did not affect them?
>>
>> Yes, it is a bug IMHO.
> 
> If it is, then it has hit no board which defines CONFIG_SPL_NAND_SIMPLE
> and we should understand why.
> 
>> Grepping for CONFIG_SPL_NAND_SIMPLE I see that TI and Tegra boards may
>> be impacted (positively or negatively):
>>
>> * CONFIG_NAND_OMAP_GPMC --- own .read_buf(), default .read_byte()
>> * CONFIG_NAND_DAVINCI   --- own .read_buf(), default .read_byte()
>> * CONFIG_TEGRA_NAND     --- own .read_byte(), own .read_buf()
> 
> They may be impacted by your change, but they are working now -- they
> are not obscure models. Let's dig a bit...

For simplicity let's neglect 16-bit NANDs at the moment, readb() can be
replaced by readw() etc. in my message.

You may notice that nand_spl_simple.c exploits NAND driver specific
.read_buf(), therefore the latter one must be defined (by driver or NAND
framework), can it happen that .read_buf(..., 1) returns a different
result from readb()?

I hope in all cases above .read_buf(..., 1) is equal to common readb(),
so okay, this is not a bug fix for currently supported drivers, but a
misfeature, which does not allow to reuse nand_spl_simple.c for a
slightly different NAND controller.

Fortunately the proposed generalization of nand_spl_simple.c is
straightforward.

>>> Conversively, what is the actual failure scenario that led you to 
>>> writing this patch?
>>
>> The failure scenario is quite simple, the ARM core gets stuck on first
>> attempt to access SLC NAND data register -- traced with JTAG.
>>
>> The same happens, if I remove custom .read_byte()/.read_buf() from SLC
>> NAND driver. The only difference between custom .read_byte() and shared
>> read_byte() is in readb()/readl() access to the data register, it is
>> essential to have only 32-bit wide access to SLC NAND registers.
> 
> README describes CONFIG_SPL_NAND_SIMPLE as "Support for NAND boot using
> simple NAND drivers that expose the cmd_ctrl() interface". The cmd_ctrl
> interface actually contains the cmd_ctrl() function *and* the
> IO_ADDR_[RW] fields. IOW, a simple NAND driver provides byte or word
> access to the NAND's I/O lines on which command, address and data are
> passed. If the NAND is 8 bits, then there are 8 lines; if it is 16
> bit, then there are 16 lines. 

Please see my note above about read_buf().

nand_spl_simple.c is designed in such a way that it uses NAND driver
specific interfaces (ECC specific interfaces are omitted here):

* cmd_ctrl()
* dev_ready()
* read_buf()

I agree that some of these interfaces may be populated by default NAND
framework, if it is a deliberate intention to have only default
functions, it might be better to hardcode default functions into
nand_spl_simple.c , but this destroys flexibility.

I would say README description of CONFIG_SPL_NAND_SIMPLE is not
precisely correct.

> I reckon that the OMAP/DAVINCI and TEGRA simple drivers just do that:
> set IO_ADDR_[RW] to the IP register through which direct access to the
> NAND's I/O lines can be performed, byte or word depending on the chip
> width.
> 
> As such, there is no bug in the way nand_simple.c uses IO_ADDR_[RW].

Okay, there is no bug in currently supported drivers, I believe.

> OTOH, the SLC IP does not provide direct access to the NAND I/O lines
> through a general register; what it provides is a set of three
> specialized registers one for commands, one for addresses and one
> for data. Plus, even though only 8 bit NANDs are supported, the data
> register does not physically support 8-bit wide accesses (NXP: I am
> still struggling to understand what you were trying to achieve there).
> 
> All this basically makes the SLC controller a 'not simple NAND IP', and
> its driver a 'not simple NAND driver' incompatible with nand_simple.c.
> 
> Your solution to this incompatibility is to change nand_simple.c to
> support other types of drivers; but by doing that, you're changing the
> API between nand_simple.c and all simple drivers, possibly changing the
> established behavior.

Could my statement be confirmed or rejected?

   One byte data read is the same if .read_buf(..., 1), .read_byte() or
readb() is used on TI and Tegra platforms, and the interfaces can be
interchanged.

I haven't tested it, but I will be surprised, if my statement is wrong.

> I personally don't think this is the right way; nand_simple.c should be
> left unchanged and the board should simply not use it since it does not
> have a simple NAND controller, and instead it should provide its own
> nand_spl_load_image().

For me an alternative change to the proposed one is to duplicate
nand_spl_simple.c functionality in LPC32xx SLC NAND driver. From
maintenance point of view this is not the best thing to do IMHO.

> But hey, I'm not then NAND custodian. Scott: your call. :)

Anyway thank you for review and comments.

--
With best wishes,
Vladimir