linux-kernel - Re: [RFC PATCH] mfd: mfd-core: inherit only valid dma

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Date:   Wed, 11 Mar 2020 14:10:04 +0100
From:   Michael Walle <michael@...le.cc>
To:     Robin Murphy <robin.murphy@....com>
Cc:     linux-kernel@...r.kernel.org, Lee Jones <lee.jones@...aro.org>,
        Christoph Hellwig <hch@....de>, Rob Herring <robh@...nel.org>,
        Russell King <linux@...linux.org.uk>
Subject: Re: [RFC PATCH] mfd: mfd-core: inherit only valid dma_masks/flags
 from parent

Am 2020-03-11 12:25, schrieb Robin Murphy:
> On 2020-03-10 11:09 pm, Michael Walle wrote:
>> Only copy the dma_masks and flags from the parent device, if the 
>> parent
>> has a valid dma_mask/flags. Commit cdfee5623290 ("driver core:
>> initialize a default DMA mask for platform device") initialize the DMA
>> masks of a platform device. But if the parent doesn't have a dma_mask
>> set, for example if it's an I2C device, the dma_mask of the child
>> platform device will be set to zero again. Which leads to many "DMA 
>> mask
>> not set" warnings, if the MFD cell has the of_compatible property set.
>> 
>> [    1.877937] sl28cpld-pwm sl28cpld-pwm: DMA mask not set
>> [    1.883282] sl28cpld-pwm sl28cpld-pwm.0: DMA mask not set
>> [    1.888795] sl28cpld-gpio sl28cpld-gpio: DMA mask not set
>> 
>> Thus a MFD child should just inherit valid dma_masks and keep the
>> platform default otherwise.
>> 
>> Cc: Christoph Hellwig <hch@....de>
>> Cc: Rob Herring <robh@...nel.org>
>> Cc: Robin Murphy <robin.murphy@....com>
>> Cc: Russell King <linux@...linux.org.uk>
>> Signed-off-by: Michael Walle <michael@...le.cc>
>> ---
>> 
>> Hi,
>> 
>> I don't know if that is the correct way of handling things. Maybe I'm
>> also doing something wrong in my driver, I had a look at other I2C MFD
>> drivers but couldn't find a clue why they shouldn't have the same
>> problem.
> 
> The underlying issue is that about 99% of MFD children should not be
> going through dma_configure() at all because their parent 'real'
> device is not on a DMA-capable bus, but as they are platform devices
> we are forced to give them the benefit of the doubt. For DT systems
> the only vaguely-reasonable heuristic to distinguish between
> "platform" meaning "SoC memory-mapped device" and "platform" meaning
> "random crap made up by Linux" is whether the device has a populated
> OF node, but MFD's trick of hanging the parent device's OF node onto
> its synthesised children kicks a hole right through even that.

Thanks for the explanation.

> Modulo any other concerns with the existing code, does the change
> below make things work the way you want? It's still a bit of a bodge,
> but short of invasive large-scale changes with bus types I don't see a
> way to do the 'right' thing :/
> 
> Robin.
> 
> ----->8-----
> diff --git a/drivers/mfd/mfd-core.c b/drivers/mfd/mfd-core.c
> index f5a73af60dd4..1e4a6e8bd630 100644
> --- a/drivers/mfd/mfd-core.c
> +++ b/drivers/mfd/mfd-core.c
> @@ -138,7 +138,7 @@ static int mfd_add_device(struct device *parent, 
> int id,
> 
>  	pdev->dev.parent = parent;
>  	pdev->dev.type = &mfd_dev_type;
> -	pdev->dev.dma_mask = parent->dma_mask;
> +	pdev->dma_mask = parent->dma_mask ? *parent->dma_mask : 0;

That works.

-michael

>  	pdev->dev.dma_parms = parent->dma_parms;
>  	pdev->dev.coherent_dma_mask = parent->coherent_dma_mask;