[U-Boot] [PATCH 6/7] getenv_f() env variable exist w/o needing a buffer

[Wolfgang Denk]

Dear James Yang,

In message <alpine.LRH.2.00.1301041608190.3906 at ra8135-ec1.am.freescale.net> you wrote:
> 
> > Hm... this adds a special case and as such increases complexity - and
> > what is the benefit for you? 
> 
> The purpose is to avoid having to allocate memory for getenv_f() to 

What exactly is the problem of adding a dynamic variable on the stack?
This is a way cheaper operation than adding the code here...

> work.  While the unmodified getenv_f() does let me do that if I pass 
> len=0, it has the side effect of printing a warning message that the 
> buffer is too small.  I want to avoid this message from being printed 
> as well.

Then just provide a big enough buffer.  You don't bother about a few
bytes of stack space, do you?  They cost you nothing...

> Part 7 of the patchset runs at a point where memory can only be 
> allocated from the stack.  The stack is in cache, so any available RAM 
> is precious.  The function that calls getenv_f() calls another 

This argument backfires - because if you detect that the variable is
set, then you will call fsl_ddr_interactive(), which then will alocate
a buffer (char buffer2[CONFIG_SYS_CBSIZE]) and call getenv_f() again,
now for real.

Actually you now need TWO such buffers - see this snippet from your
patch 7/7:

        unsigned long long ddrsize;
        const char *prompt = "FSL DDR>";
        char buffer[CONFIG_SYS_CBSIZE];
+       char buffer2[CONFIG_SYS_CBSIZE];
+       char *p = NULL;
        char *argv[CONFIG_SYS_MAXARGS + 1];     /* NULL terminated */
        int argc;
        unsigned int next_step = STEP_GET_SPD;

I. e. before one such buffer was sufficient, now you need two - if
you care about memory, then dumpt his patch, and leave the code as is
- both your code and your stack footprint will be smaller.

> function, so allocating a buffer with an unmodified getenv_f() would 
> require the buffer to persist in the calling function's stack frame 
> uselessly.  That buffer is of size CONFIG_SYS_CBSIZE, which is either 
> 256 or 1024, so I wouldn't call it non-critical.

But you do this anyway, just in another part of the code.  ANd there
you even need two such buffers now!

> I imagine that with the modified getenv_f(), other pre-relocation 
> features could be written to utilize the detection of environment 
> variables in a similar fashion.  This patch set by itself should not 
> be considered as the sole usage case.

Well, the use case you present shows that while the idea sounds good
initially, the results tend to be worse than the existing code.

You did not convince me that the addition is a good idea.

> The description was not written to be a top-down procedural 
> description.  Maybe reordering like this will make it seem more 
> correct?

This will not remove the inconsistent behaviour of returning a 1 in
one case, indepoendent of the actual length of the value, and the
length in another case.  And there is no need for such an
inconsistency.

> > > +		if (!buf)
> > > +			return 1;
> > 
> > I tend to NAK this part.
> 
> Would it be acceptable if it returns 0 instead?  The reason I chose 1 
> is because all of the 100+ existing usages of getenv_f() check only 
> for return value > 0.  I was trying to make it consistent with all of 
> those existing usage cases.

Why don't you implement consistent behaviour and always return the
correct length of the variable value, and -1 if the variable does not
exist?

Best regards,

Wolfgang Denk

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