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

["Heiko Stübner" <heiko@sntech.de>]

Am Mittwoch, 21. Oktober 2026, 04:42:39 Mitteleuropäische Sommerzeit schrieb Rob Herring:
> On Tue, Oct 20, 2026 at 04:57:34PM +0200, Heiko Stübner wrote:
> > Am Dienstag, 20. Oktober 2026, 16:10:39 Mitteleuropäische Sommerzeit schrieb Rob Herring:
> > > 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.
> > 
> > even better then ... and I didn't look deep enough it seems.
> > 
> > 
> > > > > (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?
> > 
> > wasn't the main contract, a newer kernel should support old DTs ?
> 
> Well, yes, it was. That was a position I inherited. I hear complaints 
> about the opposite though.
> 
> > Aka the DT is parked in some firmware part (u-boot?) and gets handed
> > to the newer kernel.
> > 
> > I don't see deliberately handing a newer DTB to an older kernel as a
> > very common occurence?
> 
> Do you want things to break when you do a firmware update that's picked 
> up a newer DT? I don't.
> 
> 
> > And from a practical standpoint:
> > 
> > We're talking about devices with DSI displays that are at stake here,
> > so we're in the embedded world.
> > 
> > In other areas we're still fighting basic functionality like actually
> > handling the type-c orientation correctly in the usbphy.
> > 
> > So I really don't see a device with a user-updateable mainline kernel out
> > in the field, apart from people hacking around on their devices to try
> > out new features and new kernels, simply because core functionality
> > is still evolving in mainline.
> > 
> > 
> > RK3576/RK3588 devices still ship with a vendor u-boot/kernel, with
> > completely incompatible DTB afterall.
> 
> I can only keep my own house clean...

ok :-) 


> I only ask that compatibility issues be documented. Up to individual 
> platforms to decide if they care or not. So, your decision.

The discussion would be a lot different for SoCs like the RK3399 or PX30
that can and have run mainline software in a usable way for years now.
RK3568 very likely falls in that area too.

But for the in-progress SoCs RK3576 and RK3588 anybody wanting to run
a generic userspace would need a heavily patched kernel anyway, with
other unreviewed/unmerged dt-changes, so personally I can live with it.

Especially if we keep the (in my eyes) more common case
(old dtb, new kernel) working.


Heiko



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