linux-kernel - Re: [PATCH 4/8] mfd: stm32-timers: add support for dmas

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Message-ID: <ee021839-3298-d01b-8890-619aca0399a7@st.com>
Date:   Tue, 23 Jan 2018 16:52:49 +0100
From:   Fabrice Gasnier <fabrice.gasnier@...com>
To:     Lee Jones <lee.jones@...aro.org>
CC:     <thierry.reding@...il.com>, <robh+dt@...nel.org>,
        <alexandre.torgue@...com>, <benjamin.gaignard@...aro.org>,
        <mark.rutland@....com>, <linux@...linux.org.uk>,
        <mcoquelin.stm32@...il.com>, <benjamin.gaignard@...com>,
        <devicetree@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <linux-pwm@...r.kernel.org>
Subject: Re: [PATCH 4/8] mfd: stm32-timers: add support for dmas

On 01/23/2018 04:30 PM, Lee Jones wrote:
> On Tue, 23 Jan 2018, Fabrice Gasnier wrote:
> 
>> On 01/23/2018 02:32 PM, Lee Jones wrote:
>>> On Tue, 16 Jan 2018, Fabrice Gasnier wrote:
>>>
>>>> STM32 Timers can support up to 7 dma requests:
>>>> 4 channels, update, compare and trigger.
>>>> Optionally request part, or all dmas from stm32-timers MFD core.
>>>> Also, keep reference of device's bus address to allow child drivers to
>>>> transfer data from/to device by using dma.
>>>>
>>>> Signed-off-by: Fabrice Gasnier <fabrice.gasnier@...com>
>>>> ---
>>>>  drivers/mfd/stm32-timers.c       | 37 ++++++++++++++++++++++++++++++++++++-
>>>>  include/linux/mfd/stm32-timers.h | 14 ++++++++++++++
>>>>  2 files changed, 50 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/mfd/stm32-timers.c b/drivers/mfd/stm32-timers.c
>>>> index a6675a4..372b51e 100644
>>>> --- a/drivers/mfd/stm32-timers.c
>>>> +++ b/drivers/mfd/stm32-timers.c
>>>> @@ -29,6 +29,23 @@ static void stm32_timers_get_arr_size(struct stm32_timers *ddata)
>>>>  	regmap_write(ddata->regmap, TIM_ARR, 0x0);
>>>>  }
>>>>  
>>>> +static void stm32_timers_dma_probe(struct device *dev,
>>>> +				   struct stm32_timers *ddata)
>>>> +{
>>>> +	int i;
>>>> +	char name[4];
>>>> +
>>>> +	for (i = STM32_TIMERS_DMA_CH1; i <= STM32_TIMERS_DMA_CH4; i++) {
>>>> +		snprintf(name, ARRAY_SIZE(name), "ch%1d", i + 1);
>>>> +		ddata->dmas[i] = dma_request_slave_channel(dev, name);
>>>
>>> And if any of them fail?
>>
>> Hi Lee,
>>
>> If some of these fails, reference will be NULL. It is checked in child
>> driver (pwm for instance) at runtime. Support is being added as an
>> option: pwm capture will simply be unavailable in this case (fail with
>> error).
> 
> Can you place a simple one-line comment in here please.  Or else
> someone will come along and add error handling for you.

Hi Lee,

Ok, I'll add a comment in v2.

> 
>>>> +	}
>>>> +	ddata->dmas[STM32_TIMERS_DMA_UP] = dma_request_slave_channel(dev, "up");
>>>> +	ddata->dmas[STM32_TIMERS_DMA_TRIG] =
>>>> +		dma_request_slave_channel(dev, "trig");
>>>> +	ddata->dmas[STM32_TIMERS_DMA_COM] =
>>>> +		dma_request_slave_channel(dev, "com");
>>>
>>> Can you format these in the same why.  This hurts my eyes.
>>
>> I use enum values and try to match as possible with reference manual for
>> "up", "trig" & "com" names.
>> Would have some suggestion to beautify this?
> 
> I just mean, can you push the first  dma_request_slave_channel() call
> on to the next line, so each of the calls are formatted in the same
> manner please?

Ok, got it now. I'll do this in v2.

> 
>>>> +}
>>>> +
>>>>  static int stm32_timers_probe(struct platform_device *pdev)
>>>>  {
>>>>  	struct device *dev = &pdev->dev;
>>>> @@ -44,6 +61,7 @@ static int stm32_timers_probe(struct platform_device *pdev)
>>>>  	mmio = devm_ioremap_resource(dev, res);
>>>>  	if (IS_ERR(mmio))
>>>>  		return PTR_ERR(mmio);
>>>> +	ddata->phys_base = res->start;
>>>
>>> What do you use this for?
>>
>> This is used in in child driver (pwm) for capture data transfer by dma.
> 
> Might be worth being clear about that.
> 
> Perhaps pass in 'dma_base' (phys_base + offset) instead?
> 

I guess you've had a look at [PATCH 5/8] pwm: stm32: add capture
support. Are you talking about passing phys_base + TIM_DMAR ?

If this is the case, I'd prefer to keep phys base only if you don't
mind, and handle TIM_DMAR offset in pwm driver. This way, all dma slave
config is kept locally at one place.
Or do you mean something else ?

Maybe I can add a comment here about this ?
Something like:
/* phys_base to be used by child driver, e.g. DMA burst mode */

Please advise,
Thanks,
Fabrice