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Subject: Re: [PATCH] iio: ad_sigma_delta: Don't put SPI transfer buffer on the
 stack
To:     Alexandru Ardelean <ardeleanalex@gmail.com>
Cc:     Alexandru Ardelean <alexandru.ardelean@analog.com>,
        linux-iio <linux-iio@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Jonathan Cameron <jic23@kernel.org>
References: <20201112091050.84991-1-alexandru.ardelean@analog.com>
 <49aea6de-9084-d5a2-a6be-967196570830@metafoo.de>
 <CA+U=DspP1NfHm2XyOc-Vqq=bzVea30K5pDoARUjw9H1zvst=Zw@mail.gmail.com>
From:   Lars-Peter Clausen <lars@metafoo.de>
Message-ID: <0063409a-317f-56c0-6123-b23804ab45d9@metafoo.de>
Date:   Thu, 12 Nov 2020 11:52:42 +0100
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On 11/12/20 11:14 AM, Alexandru Ardelean wrote:
> On Thu, Nov 12, 2020 at 11:55 AM Lars-Peter Clausen <lars@metafoo.de> wrote:
>> On 11/12/20 10:10 AM, Alexandru Ardelean wrote:
>>> From: Lars-Peter Clausen <lars@metafoo.de>
>>>
>>> Use a heap allocated memory for the SPI transfer buffer. Using stack memory
>>> can corrupt stack memory when using DMA on some systems.
>>>
>>> This change adds 4 bytes at the end of the current DMA buffer, which will
>>> be used by the trigger handler.
>>> This is required because the first 4 bytes are reserved for register data.
>>>
>>> Fixes: af3008485ea03 ("iio:adc: Add common code for ADI Sigma Delta devices")
>>> Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
>>> Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
>>> ---
>>>    drivers/iio/adc/ad_sigma_delta.c       | 4 ++--
>>>    include/linux/iio/adc/ad_sigma_delta.h | 2 +-
>>>    2 files changed, 3 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/drivers/iio/adc/ad_sigma_delta.c b/drivers/iio/adc/ad_sigma_delta.c
>>> index 86039e9ecaca..33297f26508a 100644
>>> --- a/drivers/iio/adc/ad_sigma_delta.c
>>> +++ b/drivers/iio/adc/ad_sigma_delta.c
>>> @@ -395,11 +395,11 @@ static irqreturn_t ad_sd_trigger_handler(int irq, void *p)
>>>        struct iio_poll_func *pf = p;
>>>        struct iio_dev *indio_dev = pf->indio_dev;
>>>        struct ad_sigma_delta *sigma_delta = iio_device_get_drvdata(indio_dev);
>>> +     uint8_t *data = &sigma_delta->data[4];
>>>        unsigned int reg_size;
>>>        unsigned int data_reg;
>>> -     uint8_t data[16];
>>>
>>> -     memset(data, 0x00, 16);
>>> +     memset(data, 0x00, 4);
>> Younger me didn't know what he was doing, this is wrong. We need the
>> extra space for the padding and timestamp.
>>
>> We also can't put the beginning of the buffer at an 4 byte offset since
>> it needs to be 8 byte aligned for the timestamp.
> I'll correct this.
> I was re-spinning this out of some old patches and discussions on this
> that I have.
> So, then this becomes 24 bytes? Or 16?
>
> Something like:
> uint8_t                         data[24] ____cacheline_aligned;
>
> uint8_t *data = &sigma_delta->data[8];

Yes.