linux-kernel - Re: [RFC 07/47] mtd: nand: stm_nand_bch: initialise the BCH Controller

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Date:	Wed, 30 Apr 2014 13:29:46 +0100
From:	Lee Jones <lee.jones@...aro.org>
To:	"Gupta, Pekon" <pekon@...com>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kernel@...inux.com" <kernel@...inux.com>,
	"computersforpeace@...il.com" <computersforpeace@...il.com>,
	"linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
	"dwmw2@...radead.org" <dwmw2@...radead.org>,
	"angus.clark@...com" <angus.clark@...com>,
	"Ezequiel Garcia (ezequiel.garcia@...e-electrons.com)" 
	<ezequiel.garcia@...e-electrons.com>
Subject: Re: [RFC 07/47] mtd: nand: stm_nand_bch: initialise the BCH
 Controller

> >> >+	/* Reset and disable boot-mode controller */
> >> >+	writel(BOOT_CFG_RESET, nandi->base + NANDBCH_BOOTBANK_CFG);
> >> >+	udelay(1);
> >> >+	writel(0x00000000, nandi->base + NANDBCH_BOOTBANK_CFG);
> >>
> >> Why using 'udelay' ?
> >> Isn't there any status register which tells you that controller is reset / initialized ?
> >> Or may be polling on NANDBCH_BOOTBANK_CFG may itself give you status.
> >
> >Documenation says:
> >
> >  "The soft reset bit has to be reset to ‘0’ to de-assert the soft
> >   reset. The soft reset bit is expected to be asserted for at least
> >   one clock cycle for proper reset"
> >
> That’s the hardware way of saying that 'enable the clock before applying reset'.
> Clock is required to propagate reset-logic to flip-flops in pipeline, which do not get direct reset.
> 
> However that apart. You may safely drop udelay(1) because this 'udelay' is at
> CPU side and won't guarantee anything about clocks at your controller side.
> But I leave it to you as this delay is pretty small.

I'd like to keep it in if it's all the same to you.  The original
author is pretty competent and I like to think that it's there for a
reason - and as you rightly say, the delay is pretty small.

-- 
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
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