Re: [PATCH] remoteproc: core: Honor device tree /alias entries when assigning IDs

[Nishanth Menon]

On 16:45-20230822, Andrew Davis wrote:
> On 8/22/23 3:12 PM, Nishanth Menon wrote:
> > On 13:25-20230822, Mathieu Poirier wrote:
> > > Hi Nishanth,
> > > 
> > > On Mon, Aug 07, 2023 at 09:02:47AM -0500, Nishanth Menon wrote:
> > > > On many platforms, such as Beaglebone-AI64 with many remote
> > > > processors, firmware configurations provided by the distributions can
> > > > vary substantially depending on the distribution build's functionality
> > > > and the specific remote cores enabled in that variant. Ensuring
> > > > consistent udev rules mapping remoteproc nodes to constant remote
> > > > proc device indices across distributions (yocto, ubuntu, debian and
> > > > it's variants, ...) on a board basis can be challenging due to the
> > > > various functions of these distributions. Varied device node paths
> > > > create challenges for applications that operate on remote processors,
> > > > especially in minimal embedded systems(initrd like) that may not
> > > > have udev-like capabilities and rely on a more straightforward bare
> > > > filesystem. This challenge is similar to that faced by I2C, RTC or the
> > > > GPIO subsystems.
> > > > 
> > > 
> > > I'm puzzled by this patch.  I can see how using an alias can help in boards with
> > > various HW configuration.  That said, and as written above, FW files for remote
> > > processors can vary based on the build's functionality.  As such "remoteproc3"
> > > will reference the same HW device on all distributions but the functionality
> > > enacted by the FW may be different.  As such I don't see how an alias can help
> > > here.  Can you provide a concrete example that highlights the benefits?
> > 
> > Correct - *if* remoteproc3 is the constant node reference.
> > 
> > To take a trivial example: We ran into this issue with:
> > https://github.com/kaofishy/bbai64_cortex-r5_example/blob/main/Makefile#L28
> > 
> > remoteproc18 apparently changed numbering in a different build.
> > 
> > If remoteproc18 remained the same between different distro builds that
> > would have probably kept the userspace constant. but it does'nt. it
> > dependent purely on probe order, which does'nt let userspace remain
> > consistent.
> > 
> > Same reason and motivation to do the following:
> > https://git.beagleboard.org/beagleboard/repos-arm64/-/blob/main/bb-customizations/suite/bookworm/debian/86-remoteproc-noroot.rules
> > in one technique to do it - but that only works if all the distros
> > follow the same udev rules - and there is no reasonable way to enforce
> > that across distributions.
> > 
> 
> Enforcing distros to behave the same isn't the job of Device Tree, udev
> rules seems like a reasonable place. Anyone dealing with Linux should know
> they should not rely on kernel provided device names/numbers
> (like with disks, network interfaces, etc.).
> 
> If you want to have a path that will always work you could use:
> 
> /sys/devices/platform/bus@f4000/bus@f4000\:r5fss@78400000/78400000.r5f/remoteproc/
> 
> for the same. I don't like that it makes an ABI out of node names,
> but better than putting any more Linux configuration in DT IMHO.

aliases are there for real reasons. So lets not confuse the two. End of
the day being able to help userspace is what it does.

-- 
Regards,
Nishanth Menon
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