[PATCH 3/3] ARM: XIP kernel: store .data compressed in ROM

[Nicolas Pitre]

On Sat, 26 Aug 2017, Ard Biesheuvel wrote:

> On 25 August 2017 at 17:26, Nicolas Pitre <nicolas.pitre at linaro.org> wrote:
> > The .data segment stored in ROM is only copied to RAM once at boot time
> > and never referenced afterwards. This is arguably a suboptimal usage of
> > ROM resources.
> >
> > This patch allows for compressing the .data segment before storing it
> > into ROM and decompressing it to RAM rather than simply copying it,
> > saving on precious ROM space.
> >
> > Because global data is not available yet (obviously) we must allocate
> > decompressor workspace memory on the stack. The .bss area is used as a
> > stack area for that purpose before it is cleared. The required stack
> > frame is 9568 bytes for __inflate_kernel_data() alone, so make sure
> > the .bss is large enough to cope with that plus extra room for called
> > functions or fail the build.
> >
> > Signed-off-by: Nicolas Pitre <nico at linaro.org>
> > ---
[...]
> > +#ifdef CONFIG_XIP_DEFLATED_DATA
> > +/*
> > + * The .bss is used as a stack area for __inflate_kernel_data() whose stack
> > + * frame is 9568 bytes. Make sure it has extra room left.
> > + */
> > +ASSERT((_end - __bss_start) >= 12288, ".bss too small for CONFIG_XIP_DEFLATED_DATA")
> 
> The open coded numbers are a bit confusing here: the stack check uses
> 10240, the code actually needs >9568 bytes and here, you assert that
> there is at least 12288 space between _end and __bss_start.

The 9568 bytes is what my particular gcc version actually uses for 
__inflate_kernel_data(). That is not a tail function so some more space 
is needed. I imagine this may vary a bit from compiler versions.

The 10240 is 2.5x page size and 12288 is 3x page size. Those numbers are 
arbitrary but I had to pick something.


Nicolas