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

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Date:   Wed, 10 Oct 2018 22:21:51 +0000
From:   Trent Piepho <tpiepho@...inj.com>
To:     "linux-spi@...r.kernel.org" <linux-spi@...r.kernel.org>,
        "talel@...zon.com" <talel@...zon.com>,
        "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>
Subject: Re: [PATCH 1/2] dt-bindings: spi: dw: add cs-override property

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.