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

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Message-ID: <20191122032422.GK16867@sasha-vm>
Date:   Thu, 21 Nov 2019 22:24:22 -0500
From:   Sasha Levin <sashal@...nel.org>
To:     David Lechner <david@...hnology.com>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Sekhar Nori <nsekhar@...com>, devicetree@...r.kernel.org
Subject: Re: [PATCH AUTOSEL 4.19 087/209] ARM: dts: da850-lego-ev3: slow down
 A/DC as much as possible

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@...hnology.com>
>>
>>[ 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