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

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Date:   Tue, 22 Aug 2023 16:45:31 -0500
From:   Andrew Davis <afd@...com>
To:     Nishanth Menon <nm@...com>,
        Mathieu Poirier <mathieu.poirier@...aro.org>
CC:     Bjorn Andersson <andersson@...nel.org>,
        <linux-kernel@...r.kernel.org>, <linux-remoteproc@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        Robert Nelson <robertcnelson@...il.com>,
        Kevin Cahalan <kevinacahalan@...il.com>
Subject: Re: [PATCH] remoteproc: core: Honor device tree /alias entries when
 assigning IDs

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@...00/bus@...00\:r5fss@...00000/78400000.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.

Andrew