[PATCH 00/27] dm: Change the way sequence numbers are implemented

[Tom Rini]

On Tue, Dec 08, 2020 at 11:52:07PM +0100, Michael Walle wrote:
> Hi Simon,
> 
> Am 2020-11-30 02:53, schrieb Simon Glass:
> > At present each device has two sequence numbers, with 'req_seq' being
> > set up at bind time and 'seq' at probe time. The idea is that devices
> > can 'request' a sequence number and then the conflicts are resolved when
> > the device is probed.
> > 
> > This makes things complicated in a few cases, since we don't really know
> > (at bind time) what the sequence number will end up being. We want to
> > honour the bind-time requests if at all possible, but in fact the only
> > source of these at present is the devicetree aliases.
> > 
> > Apart from the obvious need for sequence numbers to supports U-Boot's
> > numbering on devices on the command line, the current scheme was
> > designed to:
> > 
> > - avoid calculating the sequence number until it is needed, to save
> >   execution time
> > - allow multiple devices to obtain a particular sequence number as they
> >   are probed and removed
> > - retain a record of the 'requested' sequence number even if it turns
> > out
> >   that a device could not get it (to allow debugging and retrying)
> > 
> > After some years using the current scheme it seems on balance that these
> > goals don't have as much merit as first thought. The first point would
> > be persuasive except that we end up reading the devicetree aliases at
> > bind-time anyway. So the work of resolving the sequence numbers during
> > probing is not that great. The second point hasn't really been an issue,
> > as there is typically no contention for sequence numbers (boards tend to
> > allocate them statically in the devicetree). Re the third point, we can
> > often figure out what was requested by looking at aliases, and in the
> > cases where we can't, it doesn't seem to matter much.
> > 
> > Since we have the devicetree available at bind time, we may as well just
> > use it, in the hope that the required processing will turn out to be
> > useful later (i.e. the device actually gets used). In addition, it is
> > simpler to use a single sequence number, since it avoids confusion and
> > some extra code.
> > 
> > This series moves U-Boot to use a single, bind-time sequence number. All
> > uclasses with the DM_UC_FLAG_SEQ_ALIAS flag enabled will assign sequence
> > numbers to their devices, so that as soon as a device is bound, it has a
> > sequence number. If a devicetree alias provides the number, it will be
> > used. Otherwise, during initial binding, the first free number is used.
> 
> What does "first free number mean"?
> 
> I have a device tree with the following aliases for network:
> 
> aliases {
>         ethernet0 = &enetc0;
>         ethernet1 = &enetc1;
>         ethernet2 = &enetc2;
>         ethernet3 = &enetc6;
> };
> 
> The individual devices might be disabled, depending on the board variant
> (which might also be dynamically determined during startup).
> 
> My first smoke test with this series show the following:
> 
>   uclass 32: eth
>   0   * enetc-0 @ ffd40e60, seq 0
>   1   * ax88179_eth @ ffd51f50, seq 1
> 
> Looks like the usb ethernet device will get seq 1 assigned (after "usb
> start"). Is this intended?
> 
> If so, this is a problem, because for ethernet devices, the MAC address
> is assigned according to the ethNaddr variable. And at least for this
> board (kontron_sl28) the first four are reserved for the ones with the
> alias entries. Thus I'd have expected that the usb device will get seq 4
> assigned.

I want to echo this concern.  One of the things that can be challenging
when working with devices in Linux is that it's now long accepted that
you can't rely on anything like probe order as that can and will change
randomly (seemingly).  You have to instead rely on some stable attribute
of the device.  Consistency of device numbering is a feature for us.

-- 
Tom
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