linux-kernel - Re: [PATCH 05/36] mtd: nand: omap: Move IRQ handling from GPMC to NAND driver

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Date:	Tue, 1 Jul 2014 06:16:19 -0700
From:	Tony Lindgren <tony@...mide.com>
To:	Roger Quadros <rogerq@...com>
Cc:	Mark Brown <broonie@...nel.org>, "Gupta, Pekon" <pekon@...com>,
	"dwmw2@...radead.org" <dwmw2@...radead.org>,
	"computersforpeace@...il.com" <computersforpeace@...il.com>,
	"kyungmin.park@...sung.com" <kyungmin.park@...sung.com>,
	"ezequiel.garcia@...e-electrons.com" 
	<ezequiel.garcia@...e-electrons.com>,
	"javier@...hile0.org" <javier@...hile0.org>,
	"Nori, Sekhar" <nsekhar@...com>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>,
	"linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	ivan.khoronzhuk@...com, Alexander Shiyan <shc_work@...l.ru>
Subject: Re: [PATCH 05/36] mtd: nand: omap: Move IRQ handling from GPMC to
 NAND driver

* Roger Quadros <rogerq@...com> [140701 03:13]:
> On 06/13/2014 03:08 PM, Tony Lindgren wrote:
> > * Roger Quadros <rogerq@...com> [140613 04:43]:
> >>
> >> OK. I agree about using some kind of abstraction instead of direct access.
> > 
> > Yes and like we chatted on irc, adding a syscon mapping for for
> > the NAND specific registers might do the trick here.
> 
> After looking at the syscon driver, which relies on regmap, it seems that regmap was designed for slow control busses like I2C/SPI and using it for NAND controller register access will have a significant negative impact on performance. In the NAND case the register writes are used for each NAND command cycle and the reads for ECC checks (every page).
> 
> See how much code regmap_read and regmap_mmio_read() translates to for a simple register read i.e. readl().
> http://lxr.free-electrons.com/source/drivers/base/regmap/regmap.c#L1944
> http://lxr.free-electrons.com/source/drivers/base/regmap/regmap-mmio.c#L129
> 
> So I'm not so sure of using regmap/syscon for NAND controller register access.

OK yes I agree, it's not a good solution for a constant
register access that's needed for the ECC registers.
 
> Could there be any other abstraction method of sharing the register space between GPMC and NAND driver?
> I've also added Ivan to the thread, the author of memory/ti-aemif.c driver, to check if he faced any issues with shared register access of the AEMIF/NAND registers.

If there's no common framework available for GPMC to implement,
it's best to just export few functions from gpmc.c for the ECC
calculations.

Regards,

Tony
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