Re: [PATCH] rust: add basic ELF sections parser

["Alexandre Courbot" <acourbot@nvidia.com>]

Hi Greg,

On Sat May 31, 2025 at 10:30 PM JST, Greg KH wrote:
> On Sat, May 31, 2025 at 09:33:38PM +0900, Alexandre Courbot wrote:
>> Hi Greg,
>> 
>> On Sat May 31, 2025 at 2:45 PM JST, Greg KH wrote:
>> > On Fri, May 30, 2025 at 01:10:50PM -0500, Timur Tabi wrote:
>> >> On Fri, May 30, 2025 at 10:42 AM Greg KH <gregkh@linuxfoundation.org> wrote:
>> >> >
>> >> > On Fri, May 30, 2025 at 11:34:02PM +0900, Alexandre Courbot wrote:
>> >> > > So to try to answer your question, I am not disagreeing that userspace
>> >> > > is capable of doing what we currently do in the kernel. My follow-up
>> >> > > questions to that are: how do we command userspace to do that work for
>> >> > > us when we request the firmware, how do we provide the result to the
>> >> > > kernel, and is this something that distros can adopt easily? I'm happy
>> >> > > to consider doing things this way, but would need a few pointers to look
>> >> > > into.
>> >> >
>> >> > Again, look at how your firmware for your devices in your laptop are
>> >> > loaded today.
>> >
>> > Note, I am talking about non-gpu firmare images here (wifi, usb
>> > controllers, etc.) that are using the firmware download subsystem for
>> > ages as examples of what to look at as to how to trigger a firmware
>> > image to be loaded by userspace into the device.
>> 
>> I would really appreciate it if you could point me precisely to one
>> example (a link, a function, a file) of what you are describing because
>> I'm starting to wonder whether we are talking about the same thing.
>> 
>> Previously I mentioned udev and CONFIG_FW_LOADER_USER_HELPER, but you
>> haven't confirmed whether that was what you had in mind or not. Assuming
>> that udev is involved, I tried to snoop events while a
>> `request_firwmare` call is performed using `udevadm monitor`, but that
>> revealed no event related to firmware loading. Then looking deeper into
>> the kernel documentation confirmed that the kernel does indeed a direct
>> filesystem lookup in request_firmware [1]. IOW, the kernel looks for the
>> requested file, and if it cannot find it it's game over. This matches my
>> observations with udevadm, as I tried requesting a non-existing file and
>> no uevent was generated. I don't see what user-space can do here.
>> 
>> I also tried to look up this "firmware download subsystem" you
>> mentioned, but couldn't find anything under that name - I suspect you
>> are talking about the sysfs loading mechanism, but AFAIU this depends on 
>> CONFIG_FW_LOADER_USER_HELPER which doesn't seem to be widely enabled
>> (not on my distro at least).
>
> Yes, that is what I am referring to, as you all seem to want to do
> "complex things without a specific filename choosen".  Look at
> Documentation/driver-api/firmware/fallback-mechanisms.rst for the
> details there.
>
> Or, better yet, just have your driver name all of the individual files
> that must be loaded and then no userspace things are needed.  That "big"
> firmware file will have already been split up into the different parts
> when you write it out to the filesystem, so no need to parse anything.
>
> If this isn't going to work for some reason, I think we need a better
> "this is EXACTLY what we need to send to the hardware for the firmware
> image(s) it requires" as I'm totally confused based on the different
> people talking on this thread about totally different hypotheticals
> (i.e. 12 line elf parsers in C vs. a giant elf parser in rust, random
> hypothetical hardware values that userspace "can not know", pointing at
> obsolete crazy interfaces like remoteproc that just happen to do crazy
> things, etc.)
>
> So step back, come up with a solid design document, and let's start over
> please.

Sounds good. Let me finish evaluating all the alternatives, gather the
pros/cons of each option, and lay out the context for our needs as
clearly as possible - this should take a few days.

Cheers,
Alex.