linux-kernel - Re: [RFC PATCH 00/10] OMAP: GPMC: NAND: Introduce GPMC APIs for OMAP NAND

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Message-ID: <53BFA038.5010903@ti.com>
Date:	Fri, 11 Jul 2014 11:28:40 +0300
From:	Roger Quadros <rogerq@...com>
To:	"Gupta, Pekon" <pekon@...com>, Tony Lindgren <tony@...mide.com>
CC:	"computersforpeace@...il.com" <computersforpeace@...il.com>,
	"javier@...hile0.org" <javier@...hile0.org>,
	"ezequiel.garcia@...e-electrons.com" 
	<ezequiel.garcia@...e-electrons.com>,
	"dwmw2@...radead.org" <dwmw2@...radead.org>,
	"jg1.han@...sung.com" <jg1.han@...sung.com>,
	"Nori, Sekhar" <nsekhar@...com>,
	"linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 00/10] OMAP: GPMC: NAND: Introduce GPMC APIs for OMAP
 NAND

Hi Pekon,

On 07/11/2014 10:27 AM, Gupta, Pekon wrote:
> Hi Roger,
> 
>> From: Tony Lindgren [mailto:tony@...mide.com]
>>> * Roger Quadros <rogerq@...com> [140709 05:39]:
>>> Hi,
>>>
>>> The following hardware modules/registers are meant for NAND controller driver
>>> usage:
>>> - NAND I/O control (NAND address, data, command registers)
>>> - Prefetch/Write-post engine
>>> - ECC/BCH engine
>>>
>>> However, these registers sit in the GPMC controller's register space and there
>>> need to be some sane way to access them from the OMAP NAND controller driver.
>>>
>>> Till now the GPMC driver was populating a data structure (struct gpmc_nand_regs)
>>> with the register addresses and passing it to the OMAP NAND driver via platform
>>> data. This mechanism cannot be used for true Device tree support as custom
>>> platform data passing mechanism doesn't seem to work. Moreover, direct
>>> access to these registers must be limited to the GPMC driver. This calls for
>>> a few custom OMAP GPMC specific APIs that the OMAP NAND driver can use
>>> to access these GPMC space registers.
>>>
>>> This series attempts to add the following new APIs and gets rid of
>>> 'struct gpmc_nand_regs' and 'gpmc_update_nand_regs()'.
>>>
>>> -For NAND I/O control registers
>>> u32 omap_gpmc_read_reg(int cs, enum omap_gpmc_reg reg);
>>> void omap_gpmc_write_reg(int cs, enum omap_gpmc_reg reg, u32 val);
>>>
>>> -For Prefetch engine
>>> int omap_gpmc_prefetch_start(int cs, int fifo_th, bool dma,
>>> 			     u32 count, int is_write);
>>> int omap_gpmc_prefetch_stop(int cs);
>>> u32 omap_gpmc_get_prefetch_count(void);
>>> u32 omap_gpmc_get_prefetch_fifo_count(void);
>>>
>>> -For ECC/BCH engine
>>> void omap_gpmc_ecc_disable(void);
>>> void omap_gpmc_ecc_configure_enable(int cs, bool ecc16, u8 ecc_size0,
>>> 				    u8 ecc_size1, bool use_bch,
>>> 				    enum omap_gpmc_bch_type bch_type,
>>> 				    u8 bch_sectors, u8 bch_wrap_mode);
> 
> I think this one has too big argument list.
> And also this interface will become inconsistent when you will expand the
> NAND driver to support devices with larger page-size(like 8K NAND devices)
> Why can't we just use 
> 	omap_gpmc_write_reg(int cs, enum omap_gpmc_reg reg);
> as already defined above?

omap_gpmc_write_reg will not be optimal for reading larger result blocks
and does not create enough abstraction between the clearly separate blocks
i.e. prefetch engine and ecc engine.

> This is one-time configuration per read/write cycle so using
> 'omap_gpmc_write_reg()' shouldn't be much of issue. And this will
> automatically plugin to current chip->ecc.hwctl() calls.
> 

hwctl() doesn't take care of ECC. Those are done by
chip->ecc.correct() and chip->ecc.calculate().

> 
> 
>>> void omap_gpmc_ecc_get_result(int length, u32 *result);
> Can you please rename it to "omap_gpmc_ecc_get_hamming_result()"
> Just to keep it consistent with "omap_gpmc_ecc_get_bch_result()"
> 

OK. Sounds reasonable.

>>> void omap_gpmc_ecc_get_bch_result(int length, u8 sector, u32 *result);
>>
> This one looks good, but you should also take in int 'ecc-scheme'.

Could you please explain why?

> 
> Actually you can just move omap_calculate_ecc_bch(...) out of NAND
> driver into GPMC driver and rename it, because support of ECC
> scheme is property of hardware controller not NAND driver.
> What ecc-schemes GPMC controller supports should be inside GPMC driver,
> and NAND driver should just attach them to appropriate interfaces. Right ?
> 
> Same way just think of moving chip->ecc.hwctl() callbacks implementations
> out of NAND driver into GPMC driver. Then you would _not_ need to
> export any GPMC registers into NAND driver.

I don't agree with moving anything nand_chip related (i.e. ecc.hwctl(), ecc.calculate()
or ecc.correct()) to GMPC driver because they are supposed to be implemented by
the NAND driver. ECC is a functionality of NAND controller not of GPMC controller.
It is just a IP design mistake that they smudged the ECC registers so badly
in the GPMC register space.

Could you please explain your view point as to why you want me to move these ecc.*()
to GPMC driver other than being extensively tested?

We can extensively test the new changes before merging any code to mainline as well.

cheers,
-roger
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