From mboxrd@z Thu Jan  1 00:00:00 1970
From: Stefan Roese <sr@denx.de>
Date: Thu, 21 May 2015 18:46:40 +0200
Subject: [U-Boot] [PATCH 19/19] imx: ventana: config: enable Falcon mode
In-Reply-To: <555E0526.8020100@denx.de>
References: <1431134922-2535-1-git-send-email-tharvey@gateworks.com>	<1431134922-2535-20-git-send-email-tharvey@gateworks.com>	<CAOMZO5B_OFrQ2+2oYP6Wk8QxV4Nn3tsA9NXF_VRVBEo5BoBm3w@mail.gmail.com>
 <CAJ+vNU1nR7ERtN69K4tK8NkLwjPx-9-WN_Kt9JCASRB7GQbWXg@mail.gmail.com>
 <555E0526.8020100@denx.de>
Message-ID: <555E0BF0.3040402@denx.de>
List-Id: <u-boot.lists.denx.de>
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
To: u-boot@lists.denx.de

Hi Stefano,

On 21.05.2015 18:17, Stefano Babic wrote:
> On 19/05/2015 15:26, Tim Harvey wrote:
>
>> Note that Falcon mode also requires you have a function in the SPL
>> that decides to boot U-Boot or to skip it and boot the OS directly. In
>> the Gateworks Ventana case I didn't want to make this completely
>> dependent on a GPIO/Button because this bootloader supports about 8
>> different PCB designs in the Ventana family - some of which do not
>> have buttons or may not have buttons loaded on the board. Instead, I
>> pulled env support into the SPL and use 'boot_os' env var to decide as
>> other boards do (see spl_start_uboot()).
>
> Anyway, this can be dangerous. The reason having a GPIO (or any trigger
> from external, for example a received char from the console) is to
> switch to U-Boot if the kernel does not work (but the kernel image is
> not corrupted), mainly go to panic, and the board can be restored into
> U-Boot.
>
> If you use an environment variable and the environment is put into the
> NAND (with MMC you can change the card), there is nothing you can do
> anymore. Board tries to boot until next panic, and then again and again.

Some boards just don't have such a means as a push button or jumper to 
select the Falcon mode. That's why I introduced this environment 
variable dependent way to select Falcon mode quite some time ago for a 
PPC based board.

You are correct. If something goes completely wrong, and the board tries 
to boot into the OS with a non-working OS installed, then you are 
doomed. JTAG comes in handy then as I have experienced myself. ;)

But this is usually only while developing this feature and kernel. Once 
its tested and mature, this really works.

Again, its a trade-off since another, better method to switch between 
U-Boot- and OS- booting is not available.

Thanks,
Stefan