Message-ID: <5ee4bac4-957b-481a-8608-15886da458c2@linux.dev>
Date: Thu, 17 Jul 2025 13:12:08 -0400
From: Sean Anderson <sean.anderson@...ux.dev>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Radhey Shyam Pandey <radhey.shyam.pandey@....com>,
 Andrew Lunn <andrew+netdev@...n.ch>, "David S . Miller"
 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 netdev@...r.kernel.org, Dave Ertman <david.m.ertman@...el.com>,
 Saravana Kannan <saravanak@...gle.com>, Leon Romanovsky <leon@...nel.org>,
 linux-kernel@...r.kernel.org, Michal Simek <michal.simek@....com>,
 linux-arm-kernel@...ts.infradead.org, Ira Weiny <ira.weiny@...el.com>
Subject: Re: [PATCH net v2 1/4] auxiliary: Support hexadecimal ids

On 7/17/25 12:33, Greg Kroah-Hartman wrote:
> On Thu, Jul 17, 2025 at 12:27:44PM -0400, Sean Anderson wrote:
>> On 7/17/25 12:21, Greg Kroah-Hartman wrote:
>> > On Thu, Jul 17, 2025 at 12:04:15PM -0400, Sean Anderson wrote:
>> >> On 7/17/25 11:59, Greg Kroah-Hartman wrote:
>> >> > On Thu, Jul 17, 2025 at 11:49:37AM -0400, Sean Anderson wrote:
>> >> >> On 7/16/25 01:09, Greg Kroah-Hartman wrote:
>> >> >> > On Tue, Jul 15, 2025 at 08:01:07PM -0400, Sean Anderson wrote:
>> >> >> >> Support creating auxiliary devices with the id included as part of the
>> >> >> >> name. This allows for hexadecimal ids, which may be more appropriate for
>> >> >> >> auxiliary devices created as children of memory-mapped devices. If an
>> >> >> >> auxiliary device's id is set to AUXILIARY_DEVID_NONE, the name must
>> >> >> >> be of the form "name.id".
>> >> >> >> 
>> >> >> >> With this patch, dmesg logs from an auxiliary device might look something
>> >> >> >> like
>> >> >> >> 
>> >> >> >> [    4.781268] xilinx_axienet 80200000.ethernet: autodetected 64-bit DMA range
>> >> >> >> [   21.889563] xilinx_emac.mac xilinx_emac.mac.80200000 net4: renamed from eth0
>> >> >> >> [   32.296965] xilinx_emac.mac xilinx_emac.mac.80200000 net4: PHY [axienet-80200000:05] driver [RTL8211F Gigabit Ethernet] (irq=70)
>> >> >> >> [   32.313456] xilinx_emac.mac xilinx_emac.mac.80200000 net4: configuring for inband/sgmii link mode
>> >> >> >> [   65.095419] xilinx_emac.mac xilinx_emac.mac.80200000 net4: Link is Up - 1Gbps/Full - flow control rx/tx
>> >> >> >> 
>> >> >> >> this is especially useful when compared to what might happen if there is
>> >> >> >> an error before userspace has the chance to assign a name to the netdev:
>> >> >> >> 
>> >> >> >> [    4.947215] xilinx_emac.mac xilinx_emac.mac.1 (unnamed net_device) (uninitialized): incorrect link mode  for in-band status
>> >> >> >> 
>> >> >> >> Signed-off-by: Sean Anderson <sean.anderson@...ux.dev>
>> >> >> >> ---
>> >> >> >> 
>> >> >> >> Changes in v2:
>> >> >> >> - Add example log output to commit message
>> >> >> > 
>> >> >> > I rejected v1, why is this being sent again?
>> >> >> 
>> >> >> You asked for explanation, I provided it. I specifically pointed out why
>> >> >> I wanted to do things this way. But I got no response. So here in v2.
>> >> > 
>> >> > Again, I said, "do not do that, this is not how ids work in the driver
>> >> > model", and you tried to show lots of reasons why you wanted to do it
>> >> > this way despite me saying so.
>> >> > 
>> >> > So again, no, sorry, this isn't ok.  Don't attempt to encode information
>> >> > in a device id like you are trying to do here, that's not what a device
>> >> > id is for at all.  I need to go dig up my old patch that made all device
>> >> > ids random numbers just to see what foolish assumptions busses and
>> >> > userspace tools are making....
>> >> 
>> >> But it *is* how ids work in platform devices.
>> > 
>> > No one should ever use platform devices/bus as an excuse to do anything,
>> > it's "wrong" in so many ways, but needs to be because of special
>> > reasons.  No other bus should work like that, sorry.
>> > 
>> >> And because my auxiliary
>> >> devices are created by a platform device, it is guaranteed that the
>> >> platform device id is unique and that it will also be unique for
>> >> auxiliary devices. So there is no assumption here about the uniqueness
>> >> of any given id.
>> > 
>> > Then perhaps use the faux device api instead?
>> 
>> There's *another* pseudo bus? OK the reason why is that faux was added
>> four months ago and there is nothing under Documentation for it. So I
>> had no idea it existed. I will have a look, but perhaps you should write
>> up some documentation about why someone might want to use a "faux" bus
>> over the auxiliary bus or MFD.
> 
> "faux" is for when platform devices were being abused because someone
> just wanted a device in the device tree, and did not use any of the
> platform device resources.

OK, well that's not this. These are real devices and there may be more
than one per system. Actually, that's the primary problem I wanted to
address with this patch: you can't create more than one device with
devm_auxiliary_device_create because they will have the same id (0).

Anyway, if you really think ids should be random or whatever, why not
just ida_alloc one in axiliary_device_init and ignore whatever's
provided? I'd say around half the auxiliary drivers just use 0 (or some
other constant), which is just as deterministic as using the device
address. Another third use ida_alloc (or xa_alloc) so all that could be
removed. And the remainder just use an address of some kind (which ends
up be formatted as decimal).

--Sean

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