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Re: [avr-gcc-list] Handling __flash1 and .trampolines [Was: .trampolines

From: Erik Christiansen
Subject: Re: [avr-gcc-list] Handling __flash1 and .trampolines [Was: .trampolines location.]
Date: Wed, 12 Dec 2012 23:24:40 +1100
User-agent: Mutt/1.5.20 (2009-06-14)

Warning:  Alternative solutions are offered for some of the sub-problems.
          Choices are clearer at the end.
          The best choice depends mostly on whether one default linker
          script should handle all the use cases mooted here. 
          The reply is a bit long. (Grab a coffee ;-)

On 11.12.12 17:47, Georg-Johann Lay wrote:

[Beautifully explanatory example of AVR memory paging snipped]

> This puts x into input section .progmem1.data and expects that this section is
> located appropriately, in particular that  &x div 2^16  =  1
> It allows to use 16-bit addresses in order to address 0x10000...0x1ffff,
> similar for other __flashN.
> However, .progmem1.data is not handled by the default linker script, i.e. will
> match .progmem*

The easiest part is to stop gobbling any .progmemN.data into the .text
output section. (__flash IIUC). Just change the line:



*(.progmem.data)     /* We only want page 0 stuff here. */

There are several ways to place the .progmemN.data input sections where
we want them, one way is to just grow the "text" memory segment in the
linker MEMORY model:

  text   (rx)   : ORIGIN = 0, LENGTH = 190K        /* 3 x 64k pages */
  boot   (rx)   : ORIGIN = 62K, LENGTH = 2K
  data   (rw!x) : ORIGIN = 0x800100, LENGTH = 4K
  eeprom (rw!x) : ORIGIN = 0x810000, LENGTH = 2K

Now we tweak the end of the .text output section with:

   *(.progmem1.data)    /* Page 1 */
   *(.progmem2.data)    /* Page 2 */
   *(.progmem3.data)    /* Page 3 */
   _etext = . ;
   __code_end = . 
}  > text

(Or put the higher pages before the destructors?)

If that is done, then we have to engineer all page overflow erroring
ourselves, perhaps as described later. But we are cheating ourselves of
ld's help. If we let ld in on the secret of the memory model, then it
can detect page overflow without any effort from us. (Skip this option
if the __memx use case has to be handled as well, by the one linker script.)

We just need a separate memory segment for each physical page,
e.g. text, flash1, flash2:

  text   (rx)   : ORIGIN = 0, LENGTH = 62K
  boot   (rx)   : ORIGIN = 62K, LENGTH = 2K
  flash1 (rx)   : ORIGIN = 64K, LENGTH = 64K
  flash2 (rx)   : ORIGIN = 128K, LENGTH = 64K
  data   (rw!x) : ORIGIN = 0x800100, LENGTH = 4K       
  eeprom (rw!x) : ORIGIN = 0x810000, LENGTH = 2K

(I would though, name these physical-world entities page0, page1, page2
for user readability. There is no good reason why the .text ouput
section name has to infect the memory model. ;-) Then we would have: 

  page0  (rx)   : ORIGIN = 0, LENGTH = 62K
  boot   (rx)   : ORIGIN = 62K, LENGTH = 2K
  page1  (rx)   : ORIGIN = 64K, LENGTH = 64K
  page2  (rx)   : ORIGIN = 128K, LENGTH = 64K
  data   (rw!x) : ORIGIN = 0x800100, LENGTH = 4K       
  eeprom (rw!x) : ORIGIN = 0x810000, LENGTH = 2K

Now, instead of the tweak at the end of the .text output section, we
add new output sections after the end of that section:

.flash1 :
{  *(.progmem1.data)    /* Page 1 */
} > page1

.flash2 :
{  *(.progmem2.data)    /* Page 2 */
} > page2

Now ld will automatically detect page overflows. (But as we discover
later, there's a contrary use case which doesn't want that.)

> The preferred behavior is:
> - Locate .progmem1.data at 0x10000

A third way that can be done is by setting the VMA in an output section
used with the original memory model, e.g.:

.flash1 0x10000 :
{  *(.progmem1.data)    /* Page 1 */
} > text

It is the __memx use case which constrains our choice of method for
solving this easy matter.

> - Complain if data exceeds .progmem1.data and enters .progmem2.data

If the __memx use case does blithely overflow page boundaries without
error, so that we can't use separate memory segments and automatic
overflow detection, then the simplest linker script syntax (that I know
of) which achieves that is this assertion placed at the end of the
flash1 output section:

flash1_test = ASSERT( . < 0x1ffff, "2nd 64k page (__flash1) overflow!");

Alternatively, the following should do the same, is more explicit, and
can be placed anywhere after the section.:

   flash1_size_test = ASSERT( SIZEOF(.progmem1.data) < 0x10000, \
                      "2nd 64k page (__flash1) overflow!");

We just need to suppress these errors if __memx is used, AIUI.

> - Similar for .text

Ah, yes: .text , "1st 64k page ..."

> - Print diagnostics that are comprehensible, i.e. mention the input
> sections, that they overlap, and the symbol and object file that
> trigger the overlap.

By the time ld is locating, the input sections are not even a dim
memory, IIRC. We would be told of the overlap, and can expect the memory
segment to be named in automatic overlap detection.

In my experience the symbol and file can usually only be inferred from
examining the memory map for an input section which has suddenly grown.
The guff which falls off the end is usually the victim of shoving from
behind. Who but the designer can say which lump of the program is taking
too much room? I look at the map file in these cases, to find what has
suddenly grown obese.

> - If .text spans from 0x0 to, say, 0x2aaaa, that's fine provided
> .progmem1.data and .progmem2.data are empty.

Oh. That's quite a contrary use case, compared to what precedes. It
constrains our freedom a bit, if we want it all in one default script.
We have to _not_ use a memory segment per page, just expand "text", in
order to stop ld from automatically complaining about page overflow.
Then we can perhaps formulate such a test as:

   with_the_lot = ASSERT( SIZEOF(.text) <= 0x30000 && \ 
                  SIZEOF(.progmem1.data) == 0 && \
                  SIZEOF(.progmem2.data) == 0,
                  "text segment (__memx) overflow!");

That is compatible with separate size assertions for .progmemN.data, but
how to choose whether to allow .progmem.data to silently grow?
(__flash, __flash1 vs __memx) Use an avr-gcc commandline define (-D) to
manually select? Or use non-zero __flash1 to decide?

> The __flashN spaces were introduced with __flash and because they are not much
> more work than __flash.  I don't know if anybody is using that.

If there were no use case allowing several physical pages to be treated
as one, then all __flashN could be handled as easily as __flash1. It is
only the __memx case, which constrains our choice of linker script
design, IIUC.

> Alternative is __memx which implements 24-bit pointers.  Notice that
> address-arithmetic is still 16-bit arithmetic.  Using __memx with the code
> above, e.g.

... [example elided]

> Notice x is in .progmem.data now and an access function is used because 
> hlo8(x)
> is not known at compile time.
> This means __memx can be regarded as extension of __flash.

Yes, that's the use case which militates against expressing the flash
pages as separate memory segments. With 24-bit pointers, it is legal to
overflow physical pages? If so, the examples above which use a common
text memory segment, but separate flashN output sections, look more

> Again, it is reasonable to locate .trampolines early.  Because there
> is no limitation for .progmem.data except that it must not exceed
> 0x7fffff because higher addresses are taken as RAM or I/O locations.

So long as early trampolines are reachable by all their users, such
placement can avoid later surprises when code grows. (Having them move
hither and yon isn't my cup of tea.)

Thank you for the splendidly explanatory examples. They have helped me
understand the problem better, and have confirmed that we have solutions
for the various aspects of the problem.

I would enjoy formulating a linker script to handle the various use
cases. I have not paused tonight to update my avr-gcc, but will do so.
Then I can tweak the latest default script, and test any offering before
inflicting it on the unsuspecting. Some iteration is to be expected, and
correction of any misapprehension on my part would be gratefully


If you stew apples like cranberries, they taste more like prunes than
rhubarb does.
                                                      - Groucho Marx

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