linux-kernel - Re: [PATCH 1/2] dt-bindings: spi: dw: add cs-override property

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Message-ID: <3ad7fac8-2f13-690b-9e4b-c6b2373756b0@amazon.com>
Date:   Thu, 11 Oct 2018 10:39:22 +0300
From:   Talel Shenhar <talel@...zon.com>
To:     Trent Piepho <tpiepho@...inj.com>,
        "linux-spi@...r.kernel.org" <linux-spi@...r.kernel.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "broonie@...nel.org" <broonie@...nel.org>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "barakw@...zon.com" <barakw@...zon.com>,
        "ronenk@...zon.com" <ronenk@...zon.com>,
        "dwmw@...zon.co.uk" <dwmw@...zon.co.uk>, <jonnyc@...zon.com>
Subject: Re: [PATCH 1/2] dt-bindings: spi: dw: add cs-override property



On 10/11/2018 01:21 AM, Trent Piepho wrote:
> On Wed, 2018-10-10 at 10:08 +0300, Talel Shenhar wrote:
>> The dw spi controller has an auto-deselect of Chip-Select, in case there is
>> no data inside the Tx FIFO. While working on platforms with Alpine chips,
>> auto-deselect mode causes an issue for some spi devices that can't handle
>> the Chip-Select deselect in the middle of a transaction. It is a normal
>> behavior for a Tx FIFO to be empty in the middle of a transaction, due to
>>
> So that's the problem!  I, like everyone else I suspect, switched to
> using GPIO chip selects with this driver because of this.  I narrowed
> it down to a CS de-assert when the bus switched from TX to RX, which of
> course makes a SPI register read fail on most devices.  The TX FIFO
> would empty at this point, so that would explain it.
>
> Did the designers of this IP ever read a SPI device datasheet???
>
> Got to agree with Mark Brown, why would anyone ever want to NOT have it
> work properly?  The previous behavior is not "alternate correct", it's
> Broken.
This patch allow the Amazon changed hw to work in a correct way.
Unfortunately, the original hw doesn't support auto-deselect disable.
auto-deselect disable is a hw fixup Amazon hw engineers added on top
of the original dw IP.
The fix was to disable the auto-deselect and to allow sw
to manually control the chip-select.
This patch enables the above described Amazon hw fixup and
adds manual control over chip-select.