Re: [PATCH v7 4/9] driver: core: allow modifying device_links flags

["Nuno Sá" <noname.nuno@gmail.com>]

On Thu, 2024-01-25 at 17:57 +0100, Rafael J. Wysocki wrote:
> On Thu, Jan 25, 2024 at 4:31 PM Nuno Sá <noname.nuno@gmail.com> wrote:
> > 
> > On Thu, 2024-01-25 at 09:14 +0100, Nuno Sá wrote:
> > > 
> > > Hi Saravana,
> > > 
> > > Thanks for your feedback,
> > > 
> > > On Wed, 2024-01-24 at 19:21 -0800, Saravana Kannan wrote:
> > > > On Tue, Jan 23, 2024 at 7:14 AM Nuno Sa via B4 Relay
> > > > <devnull+nuno.sa.analog.com@kernel.org> wrote:
> > > > > 
> > > > > From: Nuno Sa <nuno.sa@analog.com>
> > > > > 
> > > > > If a device_link is previously created (eg: via
> > > > > fw_devlink_create_devlink()) before the supplier + consumer are both
> > > > > present and bound to their respective drivers, there's no way to set
> > > > > DL_FLAG_AUTOREMOVE_CONSUMER anymore while one can still set
> > > > > DL_FLAG_AUTOREMOVE_SUPPLIER. Hence, rework the flags checks to allow
> > > > > for DL_FLAG_AUTOREMOVE_CONSUMER in the same way
> > > > > DL_FLAG_AUTOREMOVE_SUPPLIER is done.
> > > > 
> > > > Curious, why do you want to set DL_FLAG_AUTOREMOVE_CONSUMER?
> > > > Especially if fw_devlink already created the link? You are effectively
> > > > trying to delete the link fw_devlink created if any of your devices
> > > > unbind.
> > > > 
> > > 
> > > Well, this is still useful in the modules case as the link will be relaxed
> > > after
> > > all devices are initialized and that will already clear AUTOPROBE_CONSUMER
> > > AFAIU. But, more importantly, if I'm not missing anything, in [1],
> > > fw_devlinks
> > > will be dropped after the consumer + supplier are bound which means I
> > > definitely
> > > want to create a link between my consumer and supplier.
> > > 
> > 
> > Ok, so to add a bit more on this, there are two cases:
> > 
> > 1) Both sup and con are modules and after boot up, the link is relaxed and
> > thus
> > turned into a sync_state_only link. That means the link will be deleted
> > anyways
> > and AUTOPROBE_CONSUMER is already cleared by the time we try to change the
> > link.
> > 
> > 2) The built-in case where the link is kept as created by fw_devlink and
> > this
> > patch effectively clears AUTOPROBE_CONSUMER.
> > 
> > Given the above, not sure what's the best option. I can think of 4:
> > 
> > 1) Drop this patch and leave things as they are. DL_FLAG_AUTOREMOVE_CONSUMER
> > is
> > pretty much ignored in my call but it will turn the link in a MANAGED one
> > and
> > clear SYNC_STATE_ONLY. I could very well just pass 0 in the flags as
> > DL_FLAG_AUTOREMOVE_CONSUMER is always ignored;
> > 
> > 2) Rework this patch so we can still change an existing link to accept
> > DL_FLAG_AUTOREMOVE_CONSUMER (in the modules case for example).
> > 
> > However, instead of clearing AUTOPROBE_CONSUMER, I would add some checks so
> > if
> > flags have one of DL_FLAG_AUTOREMOVE_SUPPLIER or DL_FLAG_AUTOREMOVE_CONSUMER
> > and
> > AUTOPROBE_CONSUMER is already set, we ignore them. In fact, right now, I
> > think
> > one could pass DL_FLAG_AUTOREMOVE_SUPPLIER and link->flags ends ups with
> > AUTOREMOVE_SUPPLIER | AUTOPROBE_CONSUMER which in theory is not allowed...
> 
> No, because DL_FLAG_AUTOREMOVE_SUPPLIER is only added to the link
> flags if DL_FLAG_AUTOREMOVE_CONSUMER is already set in there and the
> former replaces the latter.
> 

Oh yes, I missed that extra if() against the DL_FLAG_AUTOREMOVE_CONSUMER flag...

> Now, DL_FLAG_AUTOREMOVE_CONSUMER cannot be set in the link flags if
> AUTOPROBE_CONSUMER is set in there.
> 
> > 3) Keep it as-is... This one is likely a NACK as I'm getting the feeling
> > that
> > clearing stuff that might have been created by fw_devlinks is probably a no-
> > go.
> > 
> > Let me know your thoughts...
> 
> If the original creator of the link didn't indicate either
> DL_FLAG_AUTOREMOVE_CONSUMER, or DL_FLAG_AUTOREMOVE_SUPPLIER, they are
> expected to need the link to stay around until it is explicitly
> deleted.
> 
> Therefore adding any of these flags for an existing link where they
> both are unset would be a mistake, because it would effectively cause
> the link to live shorter than expected by the original creator and
> that might lead to correctness issues.
> 
> Thanks!

Thanks Rafael, your last two paragraphs make it really clear what's the
reasoning and why this patch is wrong.

Jonathan, if nothing else comes that I need a re-spin, can you drop this patch
when applying?

I think we can keep the DL_FLAG_AUTOREMOVE_CONSUMER in the device_link_add()
call as it will be ignored if fw_devlinks already created the link but might be
important if the kernel command line fw_devlink is set to 'off'.

Or maybe, as Saravan mentioned in his reply we can just pass DL_FLAG_MANAGED as
having the link around is useful in case we re-probe so we don't need to call
the consumer probe function (just to EPROBE_DEFER) without the supplier being
already there...

- Nuno Sá