[U-Boot] Strange / Unreadable console output

[Markus Hubig]

On Sat, Aug 11, 2012 at 08:15:43AM -0400, Jerry Van Baren wrote:
> On 07/27/2012 05:16 AM, Markus Hubig wrote:
> > one minor Problem I often read about in this list is some crappy / unreadable
> > console output at beginning of the U-Boot start procedure. Now I have the same
> > "visual" Problem.
> > 
> > | H�NAND:  128 MiB
> > | *** Warning - bad CRC, using default environment
> > | 
> > | In:    serial
> > | Out:   serial
> > | Err:   serial
> > | Net:   macb0
> > | macb0: Starting autonegotiation...
> > | macb0: Autonegotiation timed out (status=0x7849)
> > | macb0: link down (status: 0x7849)
> > | Hit any key to stop autoboot:  0 
> > | ...
> > 
> > Sometimes I get a good readable output, which looks like so:
> > 
> > | U-Boot 2012.04.01-00003-gab465ef-dirty (Jul 23 2012 - 14:44:29)
> > | 
> > | U-Boot code: 23F00000 -> 23F30464  BSS: -> 23F72128
> > | CPU: AT91SAM9G20
> > | Crystal frequency:   18.432 MHz
> > | CPU clock        :  396.288 MHz
> > | Master clock     :  132.096 MHz
> > | RAM Configuration:
> > | Bank #0: 20000000 64 MiB
> > | WARNING: Caches not enabled
> > | NAND:  128 MiB
> > | ...
> > 
> > Has anyone an idea how to fix this?
> 
> No. :-)

Damn! :-)

> >  Or what's the cause of it?
> 
> The UART is being mishandled, either directly or indirectly, e.g. due to
> caching issues or maybe the UART block isn't configured properly.
> 
> > Is it even related to u-boot or is it something at91bootstrap is
> > doing wrong?
> 
> Yes. :-)
> 
> Questions / observations:
> 1) When the output is garbled, is it always associated with the line
> "*** Warning - bad CRC, using default environment" and not garbled if
> the environment is OK?  That would indicate it is a configuration issue
> with the default environment.  It could also indicate a caching problem
> (see next), where your environment is obscured by an enabled cache.

No. I get the CRC Warning only after I completly delete the NAND. After
a 'saveenv' this warning is gone but my output is garbled.

> 2) The line before the garbled output becomes readable is "WARNING:
> Caches not enabled".  Does (your) at91bootstrap enable caches?  If so,
> does it disable them before jumping to u-boot?  When the output is
> garbled, does it say "WARNING: Caches not enabled" or is that line
> missing???

Hmm the only reverence I found related to caches in the at91bootstrap
code I use (https://bitbucket.org/imko/at91bootstrap) is in crt0_gnu.S.

There are some lines disabling the cache but only if I enable Thump
support, which I don't ... but I'll try this.

> A classic way for UARTs to be garbled is to inadvertently cache their
> registers.  This shows up on start up because typically caching is "all
> or nothing" until the MMU is fully configured (MMU configuration is
> complex).

OK. But is enabling the caches in context of u-boot/bootstrap generally
a good thing to do?

> When your program reads the "TxReady" flag, in this scenario it gets a
> cached value of the status register that a always says "ready" even
> though the hardware isn't ready at all.  The result is all bytes get
> immediately crammed into the Tx register, overwriting the previous byte
> and garbling it.
> 
> If you get lucky and the UART status register isn't cached (or is cached
> "not ready" but the Tx routine has a timeout so it writes the byte to
> the Tx register anyway), the UART works properly and the output isn't
> garbled.

OK good to know, I'm running a bit out of time at the moment course of
my thesis, but I'll definitely will have a look at this in detail when
I'm finished.

Cheers, Markus