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

[James Yang]

Hello Wolfgang,


On Fri, 4 Jan 2013, Wolfgang Denk wrote:

> Dear York Sun,
> 
> In message <1357323245-12455-6-git-send-email-yorksun at freescale.com> you wrote:
> > From: James Yang <James.Yang at freescale.com>
> > 
> > getenv_f() searches the environment for a variable name and copies the
> > value of the variable to a buffer pointed to by one of the function's
> > parameters.  However, this means that the buffer needs to exist and
> > needs to be of sufficient length (passed as another parameter to
> > getenv_f()) to hold the requested variable's value, even if all that is
> > desired is the mere detection of the existence of the variable itself.
> > 
> > This patch removes the requirement that the buffer needs to exist.  If
> > the pointer to the buffer is set to NULL and the requested variable is
> 
> 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 
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.


> In your code, you use this feature exactly once, which means all you
> save is a single buffer on the stack of a function that does not
> appear to be critical in terms of stack size.

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 
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.

I suppose I could create another function that only calls the 
unmodified getenv_f() and returns a boolean as to whether or not that 
variable exists so that the buffer gets deallocated as soon as the 
function returns, but it would not avoid the need to have that memory 
to be actually allocated on the stack.  Also, if the compiler inlines 
that function (this can be prevented as well), it would still make 
that memory persistent.

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.

 
> >  /*
> >   * Look up variable from environment for restricted C runtime env.
> > + * If the variable is found, return the number of bytes copied.
> > + * If buf is NULL, len is ignored, and, if the variable is found, return 1.
> > + * If the variable is not found, return -1.
> 
> I think your description is not quite correct, and I dislike the
> inconsistent behaviour we get though your patch.  So far, this
> function returns the length of the variable value, or -1 in case of
> errors.  This should not change even if we implement the suggested
> feature, i. e. even when passing NULL as buffer pointer the function
> should still return the length, and not some unrelated result.

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

> > + * If buf is NULL, len is ignored, and, if the variable is found, return 1.
> > + * If the variable is found, return the number of bytes copied.
> > + * If the variable is not found, return -1.


> > +		/* found */
> > +		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.


Regards,

--James


-- 
James Yang                    Digital Networking
James.Yang at freescale.com      Freescale Semiconductor