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

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Date:   Wed, 11 Mar 2020 08:59:27 +0100
From:   Michael Walle <michael@...le.cc>
To:     Christoph Hellwig <hch@....de>
Cc:     linux-kernel@...r.kernel.org, Lee Jones <lee.jones@...aro.org>,
        Rob Herring <robh@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        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 07:18, schrieb Christoph Hellwig:
> On Wed, Mar 11, 2020 at 12:09:35AM +0100, 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.
>> 
>> There was also a thread [1] about this topic, but there seems to be no
>> conclusion.
>> 
>> [1] https://www.spinics.net/lists/linux-renesas-soc/msg31581.html
>> 
>>  drivers/mfd/mfd-core.c | 9 ++++++---
>>  1 file changed, 6 insertions(+), 3 deletions(-)
>> 
>> diff --git a/drivers/mfd/mfd-core.c b/drivers/mfd/mfd-core.c
>> index b9eb8f40c073..5d8ea5e8e93c 100644
>> --- a/drivers/mfd/mfd-core.c
>> +++ b/drivers/mfd/mfd-core.c
>> @@ -139,9 +139,12 @@ 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->dev.dma_parms = parent->dma_parms;
>> -	pdev->dev.coherent_dma_mask = parent->coherent_dma_mask;
>> +	if (parent->dma_mask)
>> +		pdev->dev.dma_mask = parent->dma_mask;
>> +	if (parent->dma_parms)
>> +		pdev->dev.dma_parms = parent->dma_parms;
> 
> Both of these are pointers, and we can't just share them.  You need
> to allocate storage for them and the allocate the values over.

So they were alreay copied wrong before this patch? If so, should
there be a fixes patch for it? The commit who introduced the copy
dates back to 2013:
   b018e1361bad3 mfd: core: Copy DMA mask and params from parent

-michael