linux-kernel - Re: [PATCH 1/4] mfd: mc13783: Take care of semantic inversion between read and write value of two bits in POWER

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Message-ID: <4B26799F.1020507@ru.mvista.com>
Date:	Mon, 14 Dec 2009 20:45:03 +0300
From:	Sergei Shtylyov <sshtylyov@...mvista.com>
To:	Alberto Panizzo <maramaopercheseimorto@...il.com>
Cc:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>,
	Samuel Ortiz <sameo@...ux.intel.com>,
	Sascha linux-arm <s.hauer@...gutronix.de>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-arm-kernel-infradead <linux-arm-kernel@...ts.infradead.org>,
	Liam Girdwood <lrg@...mlogic.co.uk>
Subject: Re: [PATCH 1/4] mfd: mc13783: Take care of semantic inversion between
 read and write value of two bits in POWER_MISCELLANEUS register.

Alberto Panizzo wrote:
> PWGT1DRV and PWGT1DRV are two digital output controlled by two corresponding
> hardware signals (Pin PWGTnEN) that are meant to be used to control core power
> supplies.
> The register MC13783_REG_POWER_MISCELLANEOUS contain the two control and
> status bit (PWGTnSPIEN) where write and read meaning is summarised by 
> the following table:
>
>  Bit PWGTxSPIEN | Pin PWGTxEN | PWGTxDRV |  Read Back
>    0 = default  |             |          | PWGTxSPIEN
>  ---------------+-------------+----------+------------
>        1        |      x      |   Low    |     0
>        0        |      0      |   High   |     1
>        0        |      1      |   Low    |     0
>
> Writing a 1 to those bits will turn off the corresponding core
> power supply. As there is no way to read back the value of 
> PWGTnSPIEN, the behaviour chosen is to let always the hardware
> control itself leaving those bits at the default value.
>
> This patch is especially needed for manipulate the other bits
> in the same register, where the read-modify-write operation
> can produce unwanted power fault.
>
> Signed-off-by: Alberto Panizzo <maramaopercheseimorto@...il.com>
> ---
>  drivers/mfd/mc13783-core.c |   10 ++++++++++
>  1 files changed, 10 insertions(+), 0 deletions(-)
>
> diff --git a/drivers/mfd/mc13783-core.c b/drivers/mfd/mc13783-core.c
> index a1ade23..3953297 100644
> --- a/drivers/mfd/mc13783-core.c
> +++ b/drivers/mfd/mc13783-core.c
> @@ -171,6 +171,9 @@ int mc13783_reg_read(struct mc13783 *mc13783, unsigned int offset, u32 *val)
>  }
>  EXPORT_SYMBOL(mc13783_reg_read);
>  
> +#define MC13783_REG_POWER_MISCELLANEOUS	34
> +#define MC13783_REGCTRL_PWGT1SPIEN	(1 << 15)
> +#define MC13783_REGCTRL_PWGT2SPIEN	(1 << 16)
>  int mc13783_reg_write(struct mc13783 *mc13783, unsigned int offset, u32 val)
>  {
>  	u32 buf;
> @@ -187,6 +190,13 @@ int mc13783_reg_write(struct mc13783 *mc13783, unsigned int offset, u32 val)
>  
>  	buf = 1 << 31 | offset << MC13783_REGOFFSET_SHIFT | val;
>  
> +	/* Take care of table 4-24 in Freescale MC13783IGPLDRM.pdf making
> +	 * the assumption that PWGTnDRV signals controls core power supplies
> +	 * that software must not disable. */
> +	if (offset == MC13783_REG_POWER_MISCELLANEOUS)
> +		buf &= !(MC13783_REGCTRL_PWGT1SPIEN |
>   

   Are you sure ! shouldn't be ~ here?

> +			 MC13783_REGCTRL_PWGT2SPIEN);
>   

   !(MC13783_REGCTRL_PWGT1SPIEN | MC13783_REGCTRL_PWGT2SPIEN) would 
evaluate to 0 which is most probably not what you want.

WBR, Sergei

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