On Tue, 15 Jun 2021 12:44:58 +1000
David Gibson <david@xxxxxxxxxxxxxxxxxxxxx> wrote:
Hi,
> On Fri, Jun 11, 2021 at 06:10:35PM +0100, Andre Przywara wrote:
> > With -Wsign-compare, compilers warn about a mismatching signedness in
> > the different legs of the conditional operator, in fdtget.c.
> >
> > In the questionable expression, we are constructing a 16-bit value out of
> > two unsigned 8-bit values, however are relying on the compiler's
> > automatic expansion of the uint8_t to a larger type, to survive the left
> > shift. This larger type happens to be an "int", so this part of the
> > expression becomes signed.
> >
> > Fix this by explicitly blowing up the uint8_t to a larger *unsigned* type,
> > before doing the left shift. And while we are at it, convert the hardly
> > readable conditional operator usage into a sane switch/case expression.
> >
> > This fixes "make fdtget", when compiled with -Wsign-compare.
> >
> > Signed-off-by: Andre Przywara <andre.przywara@xxxxxxx>
>
> Hmm... this code is pretty confusing. I can't actually see any path
> where size will be set to something other than 4, 1 or -1.
If I read correctly, then size==2 could come from the format modifier
"h", as parsed by utilfdt_decode_type().
> The change looks correct, despite that. Though I wonder if we should
> add an fdt16_ld() helper for this case.
Done.
Cheers,
Andre
>
> > ---
> > fdtget.c | 10 ++++++++--
> > 1 file changed, 8 insertions(+), 2 deletions(-)
> >
> > diff --git a/fdtget.c b/fdtget.c
> > index 777582e..1f6eb73 100644
> > --- a/fdtget.c
> > +++ b/fdtget.c
> > @@ -62,8 +62,14 @@ static int show_cell_list(struct display_info *disp, const char *data, int len,
> > for (i = 0; i < len; i += size, p += size) {
> > if (i)
> > printf(" ");
> > - value = size == 4 ? fdt32_ld((const fdt32_t *)p) :
> > - size == 2 ? (*p << 8) | p[1] : *p;
> > + switch (size) {
> > + case 4: value = fdt32_ld((const fdt32_t *)p); break;
> > + case 2: value = ((unsigned)(*p) << 8) | p[1]; break;
> > + case 1:
> > + default:
> > + value = *p;
> > + break;
> > + }
> > printf(fmt, value);
> > }
> >
>
