Re: [PATCH AUTOSEL 4.19 087/209] ARM: dts: da850-lego-ev3: slow down A/DC as much as possible

[Sasha Levin]

On Wed, Nov 13, 2019 at 10:10:30AM -0600, David Lechner wrote:
> On 11/12/19 7:48 PM, Sasha Levin wrote:
> > From: David Lechner <david@xxxxxxxxxxxxxx>
> >
> > [ Upstream commit aea4762fb46e048c059ff49565ee33da07c8aeb3 ]
> >
> > Due to the electrical design of the A/DC circuits on LEGO MINDSTORMS EV3,
> > if we are reading analog values as fast as possible (i.e. using DMA to
> > service the SPI) the A/DC chip will read incorrect values - as much as
> > 0.1V off when the SPI is running at 10MHz. (This has to do with the
> > capacitor charge time when channels are muxed in the A/DC.)
> >
> > This patch slows down the SPI as much as possible (if CPU is at 456MHz,
> > SPI runs at 1/2 of that, so 228MHz and has a max prescalar of 256, so
> > we could get ~891kHz, but we're just rounding it to 1MHz). We also use
> > the max allowable value for WDELAY to slow things down even more.
> >
> > These changes reduce the error of the analog values to about 5mV, which
> > is tolerable.
> >
> > Commits a3762b13a596 ("spi: spi-davinci: Add support for SPI_CS_WORD")
> > and e2540da86ef8 ("iio: adc: ti-ads7950: use SPI_CS_WORD to reduce
> > CPU usage") introduce changes that allow DMA transfers to be used, so
> > this slow down is needed now.
>
> I doubt that the commits above would be backported, so it does not
> make sense to backport this patch.

I've dropped it, thanks!

--
Thanks,
Sasha