linux-kernel - Re: [PATCH 4/7] soc/tegra: pmc: Add interface to set voltage of IO rails

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Message-ID: <570E109D.6070805@nvidia.com>
Date:	Wed, 13 Apr 2016 10:25:49 +0100
From:	Jon Hunter <jonathanh@...dia.com>
To:	Laxman Dewangan <ldewangan@...dia.com>, <swarren@...dotorg.org>,
	<thierry.reding@...il.com>, <linus.walleij@...aro.org>,
	<gnurou@...il.com>, <robh+dt@...nel.org>, <mark.rutland@....com>
CC:	<linux-tegra@...r.kernel.org>, <devicetree@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <linux-gpio@...r.kernel.org>
Subject: Re: [PATCH 4/7] soc/tegra: pmc: Add interface to set voltage of IO
 rails


On 13/04/16 10:00, Laxman Dewangan wrote:
> 
> On Wednesday 13 April 2016 02:17 PM, Jon Hunter wrote:
>> On 12/04/16 15:56, Laxman Dewangan wrote:
>>> NVIDIA Tegra210 supports some of the IO interface which can operate
>>> at 1.8V or 3.3V I/O rail voltage levels. SW needs to configure
>>> Tegra PMC register to set different voltage level of IO interface based
>>> on IO rail voltage from power supply i.e. power regulators.
>>>
>>> Add APIs to set and get IO rail voltage from the client driver.
>> I think that we need some further explanation about the scenario when
>> this is used. In other words, why this configuration needs to be done in
>> the kernel versus the bootloader. Is this something that can change at
>> runtime? I could see that for SD cards it may.
> 
> Yes, SDIO3.0 support needs dynamic IO rail voltage change and so pad
> voltage change.
> 
>>>     #define GPU_RG_CNTRL            0x2d4
>>>   +static DEFINE_SPINLOCK(tegra_pmc_access_lock);
>>> +
>> We already have a mutex for managing concurrent accesses, do we need
>> this?
> Mutex is sleeping calls and we really dont need this. This is sleep for
> small duration and we should do this in spinlock.

Yes but do you need to call it from a interrupt context? It seems that
these are not called very often, may be on boot, or when swapping an SD
card, and so although a spinlock would be faster, the overhead of the
mutex would be negligible in this case. I think that you need to justify
why this needs to be a spinlock with a use-case that requires it.

>>>   +
>>> +static struct tegra_io_rail_voltage_bit_info
>>> tegra210_io_rail_voltage_info[] = {
>>> +    TEGRA_IO_RAIL_VOLTAGE(SDMMC1, 12),
>>> +    TEGRA_IO_RAIL_VOLTAGE(SDMMC3, 13),
>>> +    TEGRA_IO_RAIL_VOLTAGE(AUDIO_HV, 18),
>>> +    TEGRA_IO_RAIL_VOLTAGE(DMIC, 20),
>>> +    TEGRA_IO_RAIL_VOLTAGE(GPIO, 21),
>>> +    TEGRA_IO_RAIL_VOLTAGE(SPI_HV, 23),
>>> +};
>>> +
>> You could simply this by having a look-up table similar to what we do
>> for the powergates.
> Revising the power gate code, it needs ID matches with bit location but
> it is not the case here. We need to have lookup from ID to  bit position.

I still don't see why you could not have ...

static unsigned int tegra210_io_rail_voltage_bit[] = {
	[TEGRA_IO_RAIL_SDMMC1] = 12,
	...
}

You could avoid the for-loop in the lookup as well as all the extra
definitions. Seems a lot simpler.

> 
>>
>>>   static u32 tegra_pmc_readl(unsigned long offset)
>>>   {
>>>       return readl(pmc->base + offset);
>>> @@ -175,6 +202,16 @@ static void tegra_pmc_writel(u32 value, unsigned
>>> long offset)
>>>       writel(value, pmc->base + offset);
>>>   }
>>>   +static void _tegra_pmc_register_update(unsigned long addr,
>>> unsigned long mask,
>>> +                       unsigned long val)
>> u32 for mask and val. May be consider renaming to tegra_pmc_rmw().
> 
> update is common from the other framework like regmap so it is here.
> 
> 
>>
>>>   +static int tegra_io_rail_voltage_get_bit_pos(int io_rail_id)
>> All the tegra_io_rail_xxx() APIs just use "id" and it is an unsigned
>> int. Any reason this needs to be an int? We should keep the naming and
>> type consistent.
> 
> OK, will do.
> 
>>> +
>>> +int tegra_io_rail_voltage_set(int io_rail, int val)
>> Same here w.r.t "io_rail".
>>
>> Also it appears that "val" should only be 0 or 1 but we don't check for
>> this. I see that you treat all non-zero values as '1' but that seems a
>> bit funny. You may consider having the user pass uV and then you could
>> check for either 3300000 or 1800000.
> 
> What about enums then?
> May be we can use the DT binding header here.

Seems better just to specify the voltage in uV in the DT and pass it to
this function.

>>
>>> +
>>> +    spin_lock_irqsave(&tegra_pmc_access_lock, flags);
>>> +    _tegra_pmc_register_update(PMC_PWR_DET, mask, mask);
>>> +    _tegra_pmc_register_update(PMC_PWR_DET_VAL, mask, bval);
>> Hmmm ... this does not appear to be consistent with the TRM. It says to
>> write to the PMC_PWR_DET register and then the PMC_PWR_LATCH register. I
>> see in the Tegra210 TRM it says to only write the to ONLY the
>> PMC_PWR_DET_VAL register in other places. The TRM appears to be quite
>> confusing here, can you explain this?
> 
> The TRM needs to be update. There is no LATCH register in the T210.
> PMC_PWR_DET and PMC_PWR_DET_VAL are registers for this. I have internal
> tracking bug for correcting this.

Why do you need to program both? I think that we should be clear here
about the procedure. If the TRM is wrong, then there should be at least
a comment here describing the correct sequence.

Cheers
Jon