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

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Message-ID: <20180123164121.xjzhypqe2u5c7zza@dell>
Date:   Tue, 23 Jan 2018 16:41:21 +0000
From:   Lee Jones <lee.jones@...aro.org>
To:     Fabrice Gasnier <fabrice.gasnier@...com>
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 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?

-- 
Lee Jones
Linaro Services Technical Lead
Linaro.org │ Open source software for ARM SoCs
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