[PATCH v2] drm: rcar-du: dw-hdmi: Reject modes with a too high clock frequency

[Geert Uytterhoeven]

Hi Laurent,

On Tue, Dec 4, 2018 at 7:51 PM Laurent Pinchart
<laurent.pinchart at ideasonboard.com> wrote:
> On Tuesday, 4 December 2018 20:42:53 EET Geert Uytterhoeven wrote:
> > On Tue, Dec 4, 2018 at 7:12 PM Laurent Pinchart wrote:
> > > On Tuesday, 4 December 2018 19:30:25 EET Geert Uytterhoeven wrote:
> > >> On Tue, Dec 4, 2018 at 5:36 PM Laurent Pinchart wrote:
> > >>> Implement a .mode_valid() handler in the R-Car glue layer to reject
> > >>> modes with an unsupported clock frequency.
> > >>>
> > >>> Signed-off-by: Laurent Pinchart
> > >>> <laurent.pinchart+renesas at ideasonboard.com>
> > >>
> > >> Thanks for your patch!
> > >>
> > >>> --- a/drivers/gpu/drm/rcar-du/rcar_dw_hdmi.c
> > >>> +++ b/drivers/gpu/drm/rcar-du/rcar_dw_hdmi.c
> > >>> @@ -35,6 +35,20 @@ static const struct rcar_hdmi_phy_params
> > >>> rcar_hdmi_phy_params[] = {
> > >>>         { ~0UL,      0x0000, 0x0000, 0x0000 },
> > >>>  };
> > >>>
> > >>> +static enum drm_mode_status
> > >>> +rcar_hdmi_mode_valid(struct drm_connector *connector,
> > >>> +                    const struct drm_display_mode *mode)
> > >>> +{
> > >>> +       /*
> > >>> +        * The maximum supported clock frequency is 297 MHz, as shown
> > >>> in the PHY
> > >>> +        * parameters table.
> > >>> +        */
> > >>> +       if (mode->clock > 297000)
> > >>> +               return MODE_CLOCK_HIGH;
> > >>
> > >> Perhaps you need a check for the lower limit (25 MHz), too?
> > >
> > > There's no lower limit implied by the rcar_hdmi_phy_params table.
> >
> > Oh, you mean the table in the driver, not a table in the Hardware User's
> > Manual?
>
> Correct, I mean the table in the driver. This patch was prompted by an error
> returned from rcar_hdmi_phy_configure() when the mode frequency was too high,
> making mode setting failed. I've thus added a .mode_valid() handler to ensure
> that invalid modes don't get exposed to upper layers, fixing such use cases as
> fbvon on a 4K monitor (where the fbcon was picking a mode advertised as
> supported by the driver while its frequency was too high).
>
> > That's why I couldn't find the table, but only a short notice in the HDMI
> > section of the Hardware User's Manual, stating:
> >
> >     Pixel clock from 25MHz up to 297MHz
>
> Well, the IP core vendor doesn't allow us to submit patches based on the
> content of non-public documentation, so I'm afraid I won't sign such a patch
> without being given explicit permission. It's a very stupid game really, but I
> don't set the rules :-(

https://en.wikipedia.org/wiki/HDMI claims 25 MHz is  the minimum TMDS rate
for HDMI anyway. Anything below that needs to use pixel replication.

So you can reject < 25 MHz for sure.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds