Re: [PATCH v2 14/41] drm/modes: Move named modes parsing to a separate function

[Jani Nikula]

On Tue, 30 Aug 2022, Maxime Ripard <maxime@xxxxxxxxxx> wrote:
> Hi,
>
> On Tue, Aug 30, 2022 at 01:43:07PM +0300, Jani Nikula wrote:
>> On Tue, 30 Aug 2022, Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> wrote:
>> > On Mon, Aug 29, 2022 at 3:13 PM Maxime Ripard <maxime@xxxxxxxxxx> wrote:
>> >> +#define STR_STRICT_EQ(str, len, cmp) \
>> >> +       ((strlen(cmp) == len) && !strncmp(str, cmp, len))
>> >
>> > This is not part of the move, but newly added.
>> 
>> The same construct is also duplicated elsewhere in the series, and I
>> kept being confused by it.
>
> I'm not sure what is confusing, but I can add a comment if needed.

STR_STRICT_EQ() is what's confusing. I have to look at the
implementation to understand what it means. What does "strict" string
equality mean?

>
>> The above is precisely the same as:
>> 
>> 	str_has_prefix(str, cmp) == len
>
> Here, it's used to make sure we don't have a named mode starting with
> either e, d, or D.
>
> If I understood str_has_prefix() right, str_has_prefix("DUMB-MODE", "D")
> == strlen("DUMB-MODE") would return true, while it's actually what we
> want to avoid.

That's not true, str_has_prefix("DUMB-MODE", "D") == strlen("D") is.

> It's also used indeed in drm_get_tv_mode_from_name(), where we try to
> match a list of names with one passed as argument.
>
> With drm_get_tv_mode_from_name("NSTC", strlen("NTSC")), we would end up
> calling str_has_prefix("NTSC-J", "NTSC") == strlen("NTSC-J") which would
> work. However, we end up calling prefix not a prefix, but an entire
> string we want to match against, which is very confusing to me too.

If I get this right, you have a string and you want to check if that has
a certain prefix. Additionally, you want to check the prefix is a
certain length.

Sure, that the prefix is a certain length is more of a property of the
string, which is NUL terminated later than at length, but that's doesn't
really matter.

That condition is simply str_has_prefix(string, prefix) == length.

BR,
Jani.


-- 
Jani Nikula, Intel Open Source Graphics Center