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    SubjectRe: [PATCH v4 1/1] lib/vsprintf: Add support for printing V4L2 and DRM fourccs
    Hi Joe,

    On Tue, Nov 03, 2020 at 08:49:36AM -0800, Joe Perches wrote:
    > On Tue, 2020-11-03 at 16:56 +0200, Sakari Ailus wrote:
    > > On Tue, Nov 03, 2020 at 04:47:47PM +0200, Andy Shevchenko wrote:
    > > > On Tue, Nov 03, 2020 at 03:34:00PM +0200, Sakari Ailus wrote:
    > > > > Add a printk modifier %p4cc (for pixel format) for printing V4L2 and DRM
    > > > > pixel formats denoted by fourccs. The fourcc 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)
    > > > > +{
    > > > > + char output[sizeof("(xx)(xx)(xx)(xx) little-endian (0x01234567)")];
    > > >
    > > > I would add a comment that there is another possibility, i.e. big-endian, but
    > > > it occupies less space.
    >
    > I think it's unnecessary as it's obvious and similarly done in other
    > <foo>_string type functions.
    >
    > > > > + p = special_hex_number(p, output + sizeof(output) - 2, *fourcc,
    > > > > + sizeof(u32));
    > > >
    > > > I would go with one line here.
    > >
    > > It's wrapped since the result would be over 80 otherwise.
    >
    > Perhaps simpler as
    >
    > p = special_hex_number(p, p + 10, *fourcc, sizeof(u32));

    Yes. But having bugs elsewhere would have a magnified effect. I wouldn't be
    afraid of a newline here.

    >
    > > > The (theoretical) problem is here that the case when buffer size is not enough
    > > > to print a value will be like '(0xabc)' but should be rather '(0xabcd' like
    > > > snprintf() does in general.
    >
    > Isn't the stack buffer known to be large enough?

    Yes. If there are no bugs, that is.

    >
    > > > > + *p++ = ')';
    > > > > + *p = '\0';
    > > > > +
    > > > > + return string(buf, end, output, spec);
    >
    > Isn't the actual output buffer used here truncating output?
    >
    > If the general problem is someone using a limited length pointer
    > output like %10p4cc, then all the output is getting truncated no?

    Correct. But I'd also be surprised if anyone ever used this feature.

    --
    Kind regards,

    Sakari Ailus

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