Re: [PATCH v6 1/3] lib/vsprintf: Add support for printing V4L2 and DRM fourccs

[Sakari Ailus]

Hi Petr,

Thanks for the comments.

On Thu, Feb 11, 2021 at 06:14:28PM +0100, Petr Mladek wrote:
> On Tue 2021-02-09 19:47:55, Sakari Ailus wrote:
> > Hi Andy,
> > 
> > On Tue, Feb 09, 2021 at 11:58:40AM +0200, Andy Shevchenko wrote:
> > > On Tue, Feb 09, 2021 at 11:20:32AM +0200, Sakari Ailus wrote:
> > > > On Mon, Feb 08, 2021 at 10:43:30PM +0200, Andy Shevchenko wrote:
> > > > > On Mon, Feb 8, 2021 at 10:11 PM Sakari Ailus
> > > > > <sakari.ailus@xxxxxxxxxxxxxxx> wrote:
> > > 
> > > ...
> > > 
> > > > > > +       %p4cc   BG12 little-endian (0x32314742)
> > > > > 
> > > > > This misses examples of the (strange) escaping cases and wiped
> > > > > whitespaces to make sure everybody understands that 'D 12' will be the
> > > > > same as 'D1 2' (side note: which I disagree on, perhaps something
> > > > > should be added into documentation why).
> > > > 
> > > > The spaces are expected to be at the end only. I can add such example if
> > > > you like. There are no fourcc codes with spaces in the middle in neither
> > > > V4L2 nor DRM, and I don't think the expectation is to have them either.
> > > 
> > > But then the code suggests otherwise. Perhaps we need to extract
> > > skip_trailing_spaces() from strim() and use it here.
> > 
> > But this wouldn't affect the result in this case, would it?
> 
> Is there any existing implementation that would skip spaces, please?
> 
> IMHO, this might just hide problems. We should show exactly what
> is stored unless anyone explicitly ask for skipping that spaces.

I discussed this with Hans and we concluded spaces shouldn't be dropped.

Spaces can be present in the codes themselves in any case.

> 
> > > 
> > > ...
> > > 
> > > > > > +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)")];
> > > > > 
> > > > > Do we have any evidence / document / standard that the above format is
> > > > > what people would find good? From existing practices (I consider other
> > > > > printings elsewhere and users in this series) I find '(xx)' form for
> > > > > hex numbers is weird. The standard practice is to use \xHH (without
> > > > > parentheses).
> > > > 
> > > > Earlier in the review it was proposed that special handling of codes below
> > > > 32 should be added, which I did. Using the parentheses is apparently an
> > > > existing practice elsewhere.
> > > 
> > > Where? \xHH is quite well established format for escaping. Never heard about
> > > '(xx)' variant before this very series.
> 
> > Mauro referred to FourCC codes while reviewing an earlier version of this,
> > such as RGB(15).
> 
> This is quite easy to parse. The problem is that it is not clear
> whether it is hexa or decimal number.
> 
> > Does \× imply only the next two characters are hexadecimal? I have to admit
> > I don't remember seeting that, nor \x notation is common.
> 
> Hmm, the /xyy format might be hard to parse.
> 
> What about using the same approach as drm_get_format_name()?
> I mean printing '?' when the character is not printable.
> The exact value is printed later anyway.
> 
> The advantage is that it will always printk 4 characters.

"?" can be expanded by the shell. We (me and Hans) both though a dot (".")
would be good. These aren't going to be present in valid fourcc codes in
any case.

> 
> 
> > > > Note that neither DRM nor V4L2 currently has such fourcc codes currently.
> > > 
> > > ...
> > > 
> > > > > > +       p = special_hex_number(p, output + sizeof(output) - 2, *fourcc,
> > > > > > +                              sizeof(u32));
> > > > > 
> > > > > This is perfectly one line (in this file we have even longer lines).
> 
> Ailus, please do not take this as a criticism of your patch.
> I understand that it might have sounded like this but Andy did
> not mean it.
> 
> Andy prefers slightly longer lines over wrapping only few characters.
> It makes sense to me. There are more people with the same opinion.
> Even checkpatch.pl tolerates lines up to 100 characters these days.
> 
> Of course, this is a subsystem specific preference. You did not have
> any chance to know it. There is no need to fight over it.

Fair enough; I can violate the coding style a little in v7.

-- 
Kind regards,

Sakari Ailus