Re: [PATCH] iio: adc: max1027: allocate DMA-safe buffer

From: Marcus Folkesson
Date: Sat Dec 10 2016 - 15:59:45 EST

On Sat, Dec 10, 2016 at 05:36:34PM +0000, Jonathan Cameron wrote:
> On 09/12/16 10:24, Marcus Folkesson wrote:
> > The buffer needs to be DMA-safe when used with spi_read()
> >
> > Signed-off-by: Marcus Folkesson <marcus.folkesson@xxxxxxxxx>
> Please read the documentation in include/linux/gfp.h about GFP_DMA.
> Specifically:
> 220 * GFP_DMA exists for historical reasons and should be avoided where possible.
> 221 * The flags indicates that the caller requires that the lowest zone be
> 222 * used (ZONE_DMA or 16M on x86-64). Ideally, this would be removed but
> 223 * it would require careful auditing as some users really require it and
> 224 * others use the flag to avoid lowmem reserves in ZONE_DMA and treat the
> 225 * lowest zone as a type of emergency reserve.
> Seems unlikely this applies! This caught me by surprise as I didn't even know
> that flag existed - hence I went digging.
> Jonathan

Always learn something!
I did not know it was depricated, seems like the comment about GFP_DMA was
commited just a year ago.

I had a problem with a driver on my own that by using a non
DMA-safe buffer, so I was digging around looking for similiar cases in
the kernel.

I thought other drivers (iio/common/ssp_sensors/sspi_spi.c,
input/rmi4/rmi_spi.c) was using GFP_DMA for this purpose.

Anyway, thanks.

Marcus Folkesson