Re: spi_mpc8xxx.c: chip select polarity problem

From: Grant Likely <grant.likely@secretlab.ca>
To: Torsten Fleischer <to-fleischer@t-online.de>
Cc: spi-devel-general@lists.sourceforge.net, linuxppc-dev@lists.ozlabs.org
Subject: Re: spi_mpc8xxx.c: chip select polarity problem
Date: Thu, 26 Nov 2009 11:18:23 -0700	[thread overview]
Message-ID: <fa686aa40911261018u6c1624a7i636140707c4c0b61@mail.gmail.com> (raw)
In-Reply-To: <200911261827.38405.to-fleischer@t-online.de>

On Thu, Nov 26, 2009 at 10:27 AM, Torsten Fleischer
<to-fleischer@t-online.de> wrote:
> On Thu, Nov 26, 2009 at 13:12:04 Anton Vorontsov wrote:
> [...]
>> > Ah.  I understand what you're doing now.   Hmmm.  This approach
>> > concerns me because it relies on firmware or platform code to get CS
>> > gpios set up properly before the driver is probed.
>>
>> Yes, that was said at the very beginning of this thread.
>>
>> >  Firmware doesn't
>> > always get it right, and I prefer to avoid platform specific setups as
>> > much as possible.  Why can't the CS polarity be encoded into the
>> > device tree so the driver *does* have the polarity data at probe time?
>>
>> We have the spi-cs-high property, but it duplicates compatible
>> property. 'compatible' is enough to tell whether some device has
>> cs-high or cs-low (device's driver knows that already).
>>
>> The problem is that SPI bus master doesn't know all the devices,
>> so it can't extract that information from the compatible string.
>> To workaround that we can use 'spi-cs-high', but that's ugly
>> workaround.
>>
>> SPI modes (0,1,2,3) is another matter, some devices can work in
>> several modes, so 'spi-cpol' and 'spi-cpha' are actually useful.
>>
> To get a sane initial state the needed GPIOs can be set to be inputs during
> the driver's initialization.
> This requires pull-up/pull-down resistors connected to the chip select
> lines. I think we can assume that they exist, because the GPIOs are all inputs
> after the controller's hardware reset and the resistors are needed to have a
> well-defined voltage level on the chip select lines. Normally the level is set
> so that the devices are disabled.
>
> Therefore, it doesn't matter if the firmware sets the GPIOs wrong.

No, that's just shifting responsibility from firmware to hardware.
There is just as much broken hardware out there as broken firmware.
The assumption cannot be made that the initial state of the pin is the
inactive state of the CS line.  Plus, some GPIO pins are output only
and the inital state cannot be read.

g.

-- 
Grant Likely, B.Sc., P.Eng.
Secret Lab Technologies Ltd.