[U-Boot] [PATCH 2/9] sunxi: Move SPL s_init() code to board_init_f()

[Siarhei Siamashka]

On Sun, 1 Feb 2015 09:29:53 -0700
Simon Glass <sjg at chromium.org> wrote:

> Hi,
> 
> On 30 January 2015 at 10:53, Siarhei Siamashka
> <siarhei.siamashka at gmail.com> wrote:
> > On Mon, 29 Dec 2014 09:15:36 -0700
> > Simon Glass <sjg at chromium.org> wrote:
> >
> >> Hi Ian,
> >>
> >> On 28 December 2014 at 02:19, Ian Campbell <ijc at hellion.org.uk> wrote:
> >> > On Tue, 2014-12-23 at 12:04 -0700, Simon Glass wrote:
> >> >
> >> >> +void board_init_f(ulong dummy)
> >> >> +{
> >> > [...]
> >> >> +     /* Clear the BSS. */
> >> >> +     memset(__bss_start, 0, __bss_end - __bss_start);
> >> >> +
> >> >> +     board_init_r(NULL, 0);
> >> >
> >> > The previous (__weak) version of board_init_f also sets gd, which you've
> >> > also removed from s_init here and not added back anywhere (indeed, this
> >> > is the point...). But where is gd initialised now?
> >>
> >> It's still in start.S, I've just removed this duplicate.
> >>
> >> >
> >> > The patch generally looks good, two quick questions: has it been tested
> >> > in both FEL and regular mode, and has it been tested with a "legacy" as
> >> > well as a driver model system? (I might be able to find time in a day or
> >> > two to answer these myself, but for now I'll just ask).
> >>
> >> I haven't tried FEL, I only just heard of it in your email. I'll see
> >> if I can figure out how to test that.
> >
> > Just like Ian suspected, this patch has messed up the FEL boot mode
> > support.
> >
> > In a nutshell, FEL is a special USB protocol (accessible on a USB OTG
> > connector), which is implemented by the boot ROM and activated by
> > holding a special hardware button pressed and rebooting the device.
> > FEL supports commands to read/write device RAM and execute code on
> > the device. It is designed for device unbricking and firmware recovery.
> 
> If I understand it correctly, this is the same function that is
> available on Exynos5, Tegra, MX6 and probably others.
> 
> >
> > In particular, the FEL boot mode support is very useful for debugging
> > u-boot and kernel problems on tablets (the SD card slot can be used
> > for the UART console, while the system is booted over a micro-USB cable
> > with the help of FEL):
> >     http://linux-sunxi.org/File:MSI_Primo81_and_MicroSD_breakout.jpg
> >
> > In u-boot it is used in the following way:
> >  1. The SPL code is uploaded from the linux PC to the device SRAM via
> >     a FEL command (using the 'fel' program from sunxi-tools).
> >  2. The SPL code is executed via a FEL command and expected to
> >     initialize the DRAM controller. The code is executed as a
> >     normal C or assembly function, which needs to return control
> >     back to the BROM code when it is done. Right now this
> >     function is s_init().
> 
> Isn't this just another way of loading SPL then? What is so special /
> different about FEL?

I'm not very familiar with the other platforms, but the special part
here is "needs to return control back to the BROM code when it is done".

We are relying on the FEL USB protocol implementation, which is
hardcoded in BROM. FEL is used to upload data over USB to the device
memory. And FEL needs to be used twice. Once for uploading data to SRAM
and running the SPL. And one more time for uploading the rest of the
data to DRAM.

As we naturally can't modify the Allwinner code in BROM, here u-boot
needs to play according to the FEL rules and not the other way around.

As I see it, an alternative solution would be to implement USB OTG
support and the FEL protocol (or some other protocol) in the u-boot
SPL and stop relying on BROM code for uploading data to DRAM. Until
this is done, the sunxi FEL variant of SPL needs to return control to
the BROM code nicely after it has initialized DRAM and abstain from
doing anything else.

I hope that this clarifies the situation.

> >  3. As the DRAM is initialized and available now, the main u-boot
> >     binary is now uploaded to DRAM via FEL. Together with boot.scr,
> >     the kernel, the dtb file and optionally initramfs as needed.
> >  4. The main u-boot binary is executed via a FEL command, but never
> >     returns back to BROM anymore.
> >
> > More details are available in the linux-sunxi wiki:
> >     http://linux-sunxi.org/FEL
> >     http://linux-sunxi.org/FEL/Protocol
> >     http://linux-sunxi.org/FEL/USBBoot
> >
> > I have submitted a patch to fix this regression:
> >     https://patchwork.ozlabs.org/patch/434826/
> >
> > If you encounter problems and/or need help when testing FEL, please
> > let me know.
> 
> OK I commented on that patch.

Thanks.

-- 
Best regards,
Siarhei Siamashka