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

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Date:   Wed, 24 Jan 2018 16:30:58 +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 01/24/2018 03:56 PM, Lee Jones wrote:
> On Wed, 24 Jan 2018, Fabrice Gasnier wrote:
>> On 01/23/2018 05:41 PM, Lee Jones wrote:
>>> On Tue, 23 Jan 2018, Fabrice Gasnier wrote:
>>>> 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
>>>>>>>>  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 ?
>>>
>>> I have and I am.
>>>
>>>> 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 */
>>>
>>> I haven't seen the memory map for this device, so it's not easy for me
>>> to comment, but passing in the physical address of the parent MFD into
>>> a child device doesn't quite sit right with me.
>>>
>>> At what level does TIM_DMAR sit?  Is it a child (PWM) specific
>>> property, or is it described at parent (Timer) level?
>>>
>> Hi Lee,
>>
>> This isn't child (PWM) specific. TIM_DMAR is described at timer level as
>> well as all timers DMA requests lines. Current patchset make it useful
>> for PWM capture. Basically, I think this can be seen as interrupts, as
>> each (0..7) dma request has an enable bit (in DIER: interrupt enable
>> register). This is similar as interrupts at timer level.
>>
>> So, I understand your point regarding passing physical address of the
>> parent MFD... Speaking of interrupts, I'd probably have looked at
>> irq_chip. Regarding dma, i'm not sure what is preferred way ?
>>
>> Another way maybe to export a routine (export symbol) from MFD core, to
>> handle dma transfer from there?
>> By looking into drivers/mfd, I found similar approach, e.g.
>> rtsx_pci_dma_transfer(). Do you think this is better approach ?
>>
>> Please let me know your opinion.
> 
> It sounds fine in principle.  You are in a better position to make
> that decision as you know the H/W more intimately than I, however it
> does sound like a good idea to abstract the DMA handling from the
> device if these aren't device-{level|specific} operations.
> 

Hi Lee,

Thanks, I'll rework this in v2 to follow your advice.

Best Regards,
Fabrice