Re: [PATCH v9 09/25] drm/modes: Move named modes parsing to a separate function

[Maxime Ripard]

Hi Dave,

Happy new year :)

On Tue, Jan 02, 2024 at 03:12:26PM +0000, Dave Stevenson wrote:
> Hi Maxime
> 
> On Mon, 14 Nov 2022 at 13:00, Maxime Ripard <maxime@xxxxxxxxxx> wrote:
> >
> > The current construction of the named mode parsing doesn't allow to extend
> > it easily. Let's move it to a separate function so we can add more
> > parameters and modes.
> >
> > In order for the tests to still pass, some extra checks are needed, so
> > it's not a 1:1 move.
> >
> > Reviewed-by: Noralf Trønnes <noralf@xxxxxxxxxxx>
> > Tested-by: Mateusz Kwiatkowski <kfyatek+publicgit@xxxxxxxxx>
> > Signed-off-by: Maxime Ripard <maxime@xxxxxxxxxx>
> >
> > ---
> > Changes in v7:
> > - Add Noralf Reviewed-by
> >
> > Changes in v6:
> > - Simplify the test for connection status extras
> > - Simplify the code path to call drm_mode_parse_cmdline_named_mode
> >
> > Changes in v4:
> > - Fold down all the named mode patches that were split into a single
> >   patch again to maintain bisectability
> > ---
> >  drivers/gpu/drm/drm_modes.c | 70 +++++++++++++++++++++++++++++++++++++--------
> >  1 file changed, 58 insertions(+), 12 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/drm_modes.c b/drivers/gpu/drm/drm_modes.c
> > index 71c050c3ee6b..37542612912b 100644
> > --- a/drivers/gpu/drm/drm_modes.c
> > +++ b/drivers/gpu/drm/drm_modes.c
> > @@ -2229,6 +2229,51 @@ static const char * const drm_named_modes_whitelist[] = {
> >         "PAL",
> >  };
> >
> > +static int drm_mode_parse_cmdline_named_mode(const char *name,
> > +                                            unsigned int name_end,
> > +                                            struct drm_cmdline_mode *cmdline_mode)
> > +{
> > +       unsigned int i;
> > +
> > +       if (!name_end)
> > +               return 0;
> > +
> > +       /* If the name starts with a digit, it's not a named mode */
> > +       if (isdigit(name[0]))
> > +               return 0;
> > +
> > +       /*
> > +        * If there's an equal sign in the name, the command-line
> > +        * contains only an option and no mode.
> > +        */
> > +       if (strnchr(name, name_end, '='))
> > +               return 0;
> > +
> > +       /* The connection status extras can be set without a mode. */
> > +       if (name_end == 1 &&
> > +           (name[0] == 'd' || name[0] == 'D' || name[0] == 'e'))
> > +               return 0;
> > +
> > +       /*
> > +        * We're sure we're a named mode at this point, iterate over the
> > +        * list of modes we're aware of.
> > +        */
> > +       for (i = 0; i < ARRAY_SIZE(drm_named_modes_whitelist); i++) {
> > +               int ret;
> > +
> > +               ret = str_has_prefix(name, drm_named_modes_whitelist[i]);
> > +               if (ret != name_end)
> > +                       continue;
> > +
> > +               strcpy(cmdline_mode->name, drm_named_modes_whitelist[i]);
> > +               cmdline_mode->specified = true;
> > +
> > +               return 1;
> > +       }
> > +
> > +       return -EINVAL;
> > +}
> > +
> >  /**
> >   * drm_mode_parse_command_line_for_connector - parse command line modeline for connector
> >   * @mode_option: optional per connector mode option
> > @@ -2265,7 +2310,7 @@ bool drm_mode_parse_command_line_for_connector(const char *mode_option,
> >         const char *bpp_ptr = NULL, *refresh_ptr = NULL, *extra_ptr = NULL;
> >         const char *options_ptr = NULL;
> >         char *bpp_end_ptr = NULL, *refresh_end_ptr = NULL;
> > -       int i, len, ret;
> > +       int len, ret;
> >
> >         memset(mode, 0, sizeof(*mode));
> >         mode->panel_orientation = DRM_MODE_PANEL_ORIENTATION_UNKNOWN;
> > @@ -2306,18 +2351,19 @@ bool drm_mode_parse_command_line_for_connector(const char *mode_option,
> >                 parse_extras = true;
> >         }
> >
> > -       /* First check for a named mode */
> > -       for (i = 0; i < ARRAY_SIZE(drm_named_modes_whitelist); i++) {
> > -               ret = str_has_prefix(name, drm_named_modes_whitelist[i]);
> > -               if (ret == mode_end) {
> > -                       if (refresh_ptr)
> > -                               return false; /* named + refresh is invalid */
> > +       if (!mode_end)
> > +               return false;
> 
> I'm chasing down a change in behaviour between 6.1 and 6.6, and this
> patch seems to be at least part of the cause.
> 
> Since [1] we've had the emulated framebuffer on Pi being 16bpp to save
> memory. All good.
> 
> It used to be possible to use "video=HDMI-A-1:-32" on the kernel
> command line to set it back to 32bpp.
> 
> After this patch that is no longer possible. "mode_end = bpp_off", and
> "bpp_off = bpp_ptr - name", so with bpp_ptr = name we get mode_end
> being 0. That fails this conditional.
> drm_mode_parse_cmdline_named_mode already aborts early but with no
> error if name_end / mode_end is 0, so this "if" clause seems
> redundant, and is a change in behaviour.
> 
> We do then get a second parsing failure due to the check if (bpp_ptr
> || refresh_ptr) at [2].
> Prior to this patch my video= line would get mode->specified set via
> "if (ret == mode_end)" removed above, as ret = mode_end = 0. We
> therefore didn't evaluate the conditional that now fails.
> 
> So I guess my question is whether my command line is valid or not, and
> therefore is this a regression?

It's a mess :)

Documentation/fb/modedb.rst defines the video parameter syntax as:

<xres>x<yres>[M][R][-<bpp>][@<refresh>][i][m][eDd]

And thus mandates the x and y resolution. I guess that's what I use as a
base, and thus -bpp alone would be invalid.

But then it contradicts itself some time later by documenting that
video=HDMI-1:D is ok.

I guess what you experienced was just an oversight: it was not
documented anywhere that it was valid, so we didn't really tested it
either. We should add a unit test for it and fix it.

Maxime

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