From: Torsten Fleischer <to-fleischer@t-online.de>
To: avorontsov@ru.mvista.com
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 18:27:38 +0100 [thread overview]
Message-ID: <200911261827.38405.to-fleischer@t-online.de> (raw)
In-Reply-To: <20091126121204.GA3558@oksana.dev.rtsoft.ru>
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.
next prev parent reply other threads:[~2009-11-26 17:27 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <200911161742.46663.to-fleischer@t-online.de>
[not found] ` <200911211708.47253.to-fleischer@t-online.de>
[not found] ` <fa686aa40911241633q38dc0cf8x8b23038fe2fdf10d@mail.gmail.com>
[not found] ` <200911252141.59549.to-fleischer@t-online.de>
2009-11-25 22:11 ` spi_mpc8xxx.c: chip select polarity problem Grant Likely
2009-11-26 12:12 ` Anton Vorontsov
2009-11-26 17:27 ` Torsten Fleischer [this message]
2009-11-26 18:18 ` Grant Likely
2009-11-26 18:16 ` Grant Likely
2009-11-26 18:41 ` Anton Vorontsov
2009-11-26 18:50 ` Grant Likely
2009-11-26 19:01 ` Anton Vorontsov
2009-11-26 19:17 ` Grant Likely
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