Re: Implementation of fwnode_operations :: device_get_match_data() for software nodes?

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

On Mon, Mar 25, 2024 at 04:16:27PM +0100, Herve Codina wrote:
> On Mon, 25 Mar 2024 15:41:19 +0200
> Andy Shevchenko <andriy.shevchenko@linux.intel.com> wrote:

...

> > > I agree we don't want to have multiple approaches of doing the same thing,
> > > but I debate whether I am really doing the same thing?
> > > 
> > > If software nodes are not designed to be a good fit for my kind of use
> > > case, then what are they designed for?  
> > 
> > I think the hardware should be described in the respective format. Yet, you
> > have a point that it's too verbose to the cases when we know the layout of
> > the attached (not-hotpluggable) devices.
> > 
> > There are discussions [1,2] on how to enable DT for the cases when
> > non-discoverable HW needs to be detected and enumerated.
> > 
> > I don't know which solution will eventually be accepted, but my personal
> > opinion here that we would like to distantiate from board files as much
> > as possible.
> > 
> > Btw, for the internal (board files) code we may also use property to
> > go with (see how spi-pxa2xx uses that) to distinguish configurations.
> > But it might be not that straight as with driver data.
> > 
> > So far, I haven't seen the code (am I mistaken?) which makes use of driver data
> > for software nodes.
> > 
> > [1]: https://lore.kernel.org/lkml/20231128084236.157152-1-wenst@chromium.org/
> > [2]: https://lore.kernel.org/lkml/1692120000-46900-1-git-send-email-lizhi.hou@amd.com/
> > 
> > Aux topics which might not directly be related (in order of declining relevance
> > from my p.o.v.):
> > https://lore.kernel.org/lkml/20231130165700.685764-1-herve.codina@bootlin.com/
> > https://lore.kernel.org/lkml/DM6PR12MB3993D5ECA50B27682AEBE19FCD67A@DM6PR12MB3993.namprd12.prod.outlook.com/
> > https://lore.kernel.org/lkml/20240217010557.2381548-1-sboyd@kernel.org/
> > 
> 
> I work on PCI DT overlay support.
> The idea is to have a PCI driver that loads a DT overlay to describe the
> hardware embedded in the related PCI device. Some features related to this
> topic are already upstream.
> 
> Rob did a presentation of this topic at the Linux Plumber conference last
> year (https://www.youtube.com/watch?v=MVGElnZW7BQ).
> 
> IMHO, your use-case is pretty much the same. Of course it is not a PCI
> device but a SPI device. Even if the device beyond the SPI bus cannot be
> memory mapped, the idea seems pretty much the same: describe the hardware
> embedded in a specific device.
> 
> You mentioned that you need the '-@' option when you compile your base dtb.
> In fact, it depends on the resources you need to reference from your overlay.
> On my case (DT overlay to describe a PCI device), I don't need any references
> to a base dtb from the overlay. I don't need to use the '-@' option.
> Even more, I started (not yet upstream) to use all of this feature on an ACPI
> base system and it works.
> 
> My PCI device is a Microchip LAN9662 PCI device.
> The Microchip LAN9962 can be a "traditional" SoC with CPU cores and several
> IPs but also a PCI device.
> When provided as a PCI device, the internal CPU cores are no more available
> and replaced by a PCI endpoint IP.
> All the accesses done by the SoC CPU cores are replaced by accesses done by
> the host PCI system through the PCI endpoint.
> Drivers were already present upstream (traditional SoC platform driver such
> as i2c mdio, clock, switch, ...) and are used without any modifications for
> the PCI device.
> All the wiring (mapping) between the PCI world and the internal device
> hardware is done using a DT overlay.

Thank you, Herve, for looking into this. As far as I understood, slowly but
successfully the required changes for your use case are being landed. If so,
it makes driver data for software nodes approach even lower priority.

-- 
With Best Regards,
Andy Shevchenko