Re: [PATCH v5 2/8] dt-bindings: phy: rockchip,rk3588-mipi-dcphy: Add a cell to select TX or RX

[Rob Herring <robh@kernel.org>]

On Tue, Oct 20, 2026 at 03:37:35PM +0200, Heiko Stübner wrote:
> Am Dienstag, 20. Oktober 2026, 00:20:47 Mitteleuropäische Sommerzeit schrieb Michael Riesch:
> > Hi all,
> > 
> > On 10/19/26 17:06, Rob Herring wrote:
> > > On Mon, Oct 19, 2026 at 04:15:03PM +0800, 楊智成 wrote:
> > >> Hi Krzysztof, Rob,
> > >>
> > >>> Existing OS should ignore surplus cell if the driver is coded properly.
> > >>> Same as adding new properties or new front-compatible.  The question is
> > >>> how the existing driver behaves?
> > >>
> > >> As you pointed out on 5/8, the xlate checks args_count != 1. The DC-PHY
> > >> driver was merged in v6.15, so every kernel since 6.15 fails on a second cell.
> > >>
> > >> 1. Keep the second cell. Add the patch as follows:
> > >>
> > >> - if (args->args_count != 1) {
> > >> + if (args->args_count < 1) {
> > >>     dev_err(dev, "invalid number of arguments\n");
> > >>     return ERR_PTR(-EINVAL);
> > >>   }
> > >>
> > >> + if (args->args_count > 1 && args->args[1]) {
> > >> +   dev_err(dev, "only the transmitter is supported\n");
> > >> +   return ERR_PTR(-EINVAL);
> > >> + }
> > >>
> > >>   With Fixes: and Cc: stable, the stable kernels maintained at that
> > >>   time (currently 6.18.y and 7.2.y) keep DSI working with the new
> > >>   dts, and the receiver fails cleanly instead of getting the transmitter.
> > >>   But EOL kernels (6.15, 6.16, 6.17, 6.19, 7.0, 7.1) stay affected.
> > >>   It goes in with the series, since two-cell dts only exist after it.
> > >>
> > >> 2. Use a flag in the upper bits of the existing cell, as Rob
> > >>     suggested. Existing kernels are not affected, unless a board enables
> > >>     the DC-PHY CSI-2 receiver on 7.0 or later without this series.
> > >>
> > >> I will write the impact clearly whichever  approach is selected in
> > >> the related commit messages and the cover letter.
> > >>
> > >> I think the cell approach is easier to read and more symmetric in the
> > >> long term,
> > >> but it breaks more. Which one do you think is better? Thanks.
> > > 
> > > I'm not a user of this platform, so I don't care. Up to the platform 
> > > maintainers to decide if breakage is acceptible. You just need to 
> > > document the issue in the commit message.
> > > 
> > > Rob
> > 
> > It's a bit of a pickle, all this.
> > 
> > I gave it some more thought. In the end this is an IP block with two
> > PHYs. No more, no less. One cell is sufficient to address those, and it
> > could be 0 for the transmitting PHY (DSI, display) and 1 for the
> > receiving PHY (CSI, camera).
> > 
> > Of course the two PHYs can operate in either DPHY or CPHY mode. The
> > phy-cells have been (mis?)used to select the proper mode of combo PHYs,
> > and this principle has been applied here too.
> > 
> > However, we have the special situation here that the mode can only be
> > changed for the complete IP block (and thus for both PHYs at the same
> > time). As Bryan points out, we need to check and complain if the PHY
> > mode is not set to the same value for both PHYs.
> > 
> > I suggest using a property
> >     samsung,cphy-mode;
> 
> I guess, a maybe nicer way would be mimicing
> 	Documentation/devicetree/bindings/phy/intel,combo-phy.yaml
> 
> with its
> 	intel,phy-mode = <x>
> 
> with x coming from include/dt-bindings/phy/phy.h
> for the distinction between cphy and dphy, so we don't introduce yet
> another way to describe this.

We already have the plain 'phy-type' property which takes the same 
PHY_MODE_x values. We don't need yet another vendor specific property.

> 
> > (name to be bikeshedded) that sets the complete IP block into CPHY mode
> > (DPHY being the default) to distinguish between the modes. Since both
> > modes use a different electrical interface, this is clearly a hardware
> > thing and thus suitable for device tree.
> > 
> > Then, we can address the two PHYs with the help of the already existing
> > phy cell (without any bitflag tweaks to the value).
> > 
> > The sole drawback is that we need to accept the PHY_TYPE_DPHY and
> > PHY_TYPE_CPHY values that some OS in the field might pass. (For Linux,
> > it should only be the former.) And we need to handle those values
> > correctly. But I think this is not more painful than the other
> > approaches. At the same time, this design seems to be the most correct
> 
> The current cell values are 10 + 11, so as long as we don't re-use those
> exact values for the other constants the driver should somewhat easily
> be able to recognize an old DT?

I think you will still have an ABI issue of what does old OS do with new 
values?

Rob