linux-kernel - Re: [PATCH v6 1/8] drm: bridge: dw_mipi_dsi: add initial regmap infrastructure

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Message-ID: <CAFqH_513KB+En_xbpXSBG6Q38kYxWCgw0KO3NVxCb6fqHDaKBA@mail.gmail.com>
Date:   Thu, 16 Apr 2020 22:15:19 +0200
From:   Enric Balletbo Serra <eballetbo@...il.com>
To:     Adrian Ratiu <adrian.ratiu@...labora.com>
Cc:     Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        Adrian Pop <pop.adrian61@...il.com>,
        Jonas Karlman <jonas@...boo.se>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Collabora Kernel ML <kernel@...labora.com>,
        Ezequiel Garcia <ezequiel@...labora.com>,
        linux-stm32@...md-mailman.stormreply.com,
        Arnaud Ferraris <arnaud.ferraris@...labora.com>,
        linux-imx@....com
Subject: Re: [PATCH v6 1/8] drm: bridge: dw_mipi_dsi: add initial regmap infrastructure

Hi Adrian,

[snip]

> >>
> >> +static void dw_mipi_dsi_get_hw_version(struct dw_mipi_dsi
> >> *dsi) +{ +       regmap_read(dsi->regs, DSI_VERSION,
> >> &dsi->hw_version); +       dsi->hw_version &= VERSION; +
> >> if (!dsi->hw_version) +               dev_err(dsi->dev, "Failed
> >> to read DSI hw version register\n");
> >
> > Is this an error that should be ignored? If you can't get the HW
> > version, probably, there is something wrong with your hardware
> > so, don't you need to return an error?
> >
>
> After thinking a bit more about it, that error should be a
> warning.
>
> I added it because in some cases (for eg. if the peripheral clock
> is disabled) the reads can return 0 which is obviously an invalid
> version and the bridge will error in the next step when not
> finding a layout.
>

If you'll error anyway, why wait? IIUC at this point the clock *must*
be enabled, and if not, something is wrong with the driver, I don't
see any advantage on delay the error. do you have a use case where
this is called and peripheral clock disabled?

> So I'll make this a warning in v7 and explicitely mention that
> reads version == 0 can be caused by a disabled pclk.
>

-- Enric