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SubjectRe: [PATCH v2 1/1] lib/vsprintf: Add support for printing V4L2 and DRM fourccs
Hi Andy,

On Fri, Apr 03, 2020 at 12:54:41PM +0300, Andy Shevchenko wrote:
> On Fri, Apr 03, 2020 at 12:39:16PM +0300, Sakari Ailus wrote:
> > On Fri, Apr 03, 2020 at 12:31:03PM +0300, Andy Shevchenko wrote:
> > > On Fri, Apr 03, 2020 at 12:11:56PM +0300, Sakari Ailus wrote:
> > > > Add a printk modifier %ppf (for pixel format) for printing V4L2 and DRM
> > > > pixel formats denoted by 4ccs. The 4cc encoding is the same for both so
> > > > the same implementation can be used.
> > >
> > > ...
> > >
> > > > +static noinline_for_stack
> > > > +char *fourcc_string(char *buf, char *end, const u32 *fourcc,
> > > > + struct printf_spec spec, const char *fmt)
> > > > +{
> > >
> > > > +#define FOURCC_STRING_BE "-BE"
> > > > + char s[sizeof(*fourcc) + sizeof(FOURCC_STRING_BE)] = { 0 };
> > >
> > > I guess it makes it too complicated.
> >
> > The above also clearly binds the size to the data that is expected to
> > contain there. I'd prefer keeping it as-is. And yes, 8 would be correct,
> > too.
>
> OK.
>
> > > char s[8];
> > >
> > > > + if (check_pointer(&buf, end, fourcc, spec))
> > > > + return buf;
> > > > +
> > > > + if (fmt[1] != 'c' || fmt[2] != 'c')
> > > > + return error_string(buf, end, "(%p4?)", spec);
> > > > +
> > >
> > > > + put_unaligned_le32(*fourcc & ~BIT(31), s);
> > >
> > > Can you elaborate what the difference in output with this bit set over cleared?
> > > I.o.w. why don't we need to put it as BE and for LE case addd "-LE"?
> >
> > The established practice is that big endian formats have "-BE" suffix
> > whereas the little endian ones have nothing. (At least when it comes to
> > V4L2.)
>
> What I meant by the first part of the question is ordering of the characters.
> That ordering of characters is not related to that flag, correct? So, bit
> actually defines the endianess of the data in the certain fourcc.
>
> Probably you need to put a comment to explain this.

How about:

The 31st bit defines the endianness of the data, so save its printing to
the big endian suffix.

>
> > > > + if (*fourcc & BIT(31))
> > > > + strscpy(s + sizeof(*fourcc), FOURCC_STRING_BE,
> > > > + sizeof(FOURCC_STRING_BE));
> > >
> > > We know the size, and we may put '\0' as well
> > > if (*fourcc & BIT(31))
> > > strscpy(&s[4], "-BE", sizeof("-BE"));
> > > else
> > > strscpy(&s[4], "", sizeof(""));
> >
> > The rest of the struct memory has already been set to zero in variable
> > declaration.
>
> Which is bogus in my opinion. strscpy() or direct '\0' termination will put it
> more explicit.

There's no need to assign nul a simple character using strscpy(). In that
case I'd just do

s[sizeof(*fourcc)] = '\0';

and remove the initial assignment to zero.

--
Regards,

Sakari Ailus

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