linux-kernel - Re: [PATCH v2 2/2] phy: amlogic: Add a new driver for the HDMI TX PHY on Meson8/8b/8m2

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Date:   Thu, 22 Jul 2021 14:38:52 +0530
From:   Vinod Koul <vkoul@...nel.org>
To:     Martin Blumenstingl <martin.blumenstingl@...glemail.com>
Cc:     robh+dt@...nel.org, linux-phy@...ts.infradead.org,
        devicetree@...r.kernel.org, linux-amlogic@...ts.infradead.org,
        kishon@...com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] phy: amlogic: Add a new driver for the HDMI TX
 PHY on Meson8/8b/8m2

On 21-07-21, 00:08, Martin Blumenstingl wrote:
> Hi Vinod,
> 
> On Tue, Jul 20, 2021 at 10:37 AM Vinod Koul <vkoul@...nel.org> wrote:
> [...]
> > > +     if (clk_get_rate(priv->tmds_clk) >= 2970UL * 1000 * 1000)
> > > +             hdmi_ctl0 = 0x1e8b;
> > > +     else
> > > +             hdmi_ctl0 = 0x4d0b;
> >
> > magic numbers..? I guess these are register offsets, would be better to
> > define..
> Unfortunately these are register values, not offsets
> The only "documentation" I have is:
> - documentation for the bits/bit-fields in these registers [0]
> - some reference code with magic values from the vendor BSP: [1]
> 
> HDMI_CTL0/HDMI_CTL1 (the names from the datasheet) is not very
> specific and I could not find any other explanation on what the values
> mean.
> That's why I cannot offer more than these magic values (same situation
> for your finding below).

Ok, Can you add a comment that register documentation not available ...


> > > +     ret = device_property_read_u32_array(&pdev->dev, "reg", reg,
> > > +                                          ARRAY_SIZE(reg));
> >
> > we have reg as single property, why array with 2 entries here?
> My thought when Rob requested a "reg" property in the dt-bindings was
> that I should use offset and size.
> I am not validating the size here, which would be in reg[1].
> If it's fine for Rob as well then I'll switch the dt-bindings to just
> have the offset inside the reg property.

So the property is reg address and size. Two would imply you are using
two reg values.

So I would recommend to use:
        reg_offset = platform_get_resource(pdev, IORESOURCE_MEM, 0);

and skip this reg array.


> 
> [...]
> > > +static const struct of_device_id phy_meson8_hdmi_tx_of_match[] = {
> > > +     { .compatible = "amlogic,meson8-hdmi-tx-phy" },
> > > +     { .compatible = "amlogic,meson8b-hdmi-tx-phy" },
> > > +     { .compatible = "amlogic,meson8m2-hdmi-tx-phy" },
> > > +     { /* sentinel */ }
> >
> > I see that all three are handled similarly, no difference!
> So far this is correct, they're all treated the same.
> However, it happened to me (multiple times) in the past that later on
> I would spot a difference hidden in the vendor BSP.
> One example is commit f004be596c28f9 ("phy: amlogic: meson8b-usb2: Add
> a compatible string for Meson8m2").
> I know that other parts of the graphics pipeline are different on
> Meson8 compared to the other two SoCs (because Meson8b/Meson8m2 have
> some reset lines which need to be toggled after updating the video
> clocks. these resets don't exist on Meson8).
> So I decided to play safe and add compatible strings for every SoC so
> I can easily handle any differences in the future (in case I find
> any).

Correct, that is why you need to *keep* the SoC specific compatible and
document them. But use a generic one when you don't have any delta

Above would become:
        { .compatible = "amlogic,meson8-hdmi" },

with DTS specifying:
        compatible = "amlogic,meson8-hdmi-tx-phy", "amlogic,meson8-hdmi";

That way if required you can always use the specific one

-- 
~Vinod