Re: [PATCH v2 7/9] driver core: Set fw_devlink.strict=1 by default

[Saravana Kannan]

On Wed, Jun 22, 2022 at 12:40 PM Saravana Kannan <saravanak@xxxxxxxxxx> wrote:
>
> On Wed, Jun 22, 2022 at 1:44 AM Linus Walleij <linus.walleij@xxxxxxxxxx> wrote:
> >
> > On Wed, Jun 22, 2022 at 9:48 AM Sascha Hauer <sha@xxxxxxxxxxxxxx> wrote:
> >
> > > This patch has the effect that console UART devices which have "dmas"
> > > properties specified in the device tree get deferred for 10 to 20
> > > seconds. This happens on i.MX and likely on other SoCs as well. On i.MX
> > > the dma channel is only requested at UART startup time and not at probe
> > > time. dma is not used for the console. Nevertheless with this driver probe
> > > defers until the dma engine driver is available.
>
> FYI, if most of the drivers are built in, you could set
> deferred_probe_timeout=1 to reduce the impact of this (should drop
> down to 1 to 2 seconds). Is that an option until we figure out
> something better?
>
> Actually, why isn't earlyconsole being used? That doesn't get blocked
> on anything and the main point of that is to have console working from
> really early on.
>
> > >
> > > It shouldn't go in as-is.
> >
> > This affects all machines with the PL011 UART and DMAs specified as
> > well.
> >
> > It would be best if the console subsystem could be treated special and
> > not require DMA devlink to be satisfied before probing.
>
> If we can mark the console devices somehow before their drivers probe
> them, I can make fw_devlink give them special treatment. Is there any
> way I could identify them before their drivers probe?
>
> > It seems devlink is not quite aware of the concept of resources that are
> > necessary to probe vs resources that are nice to have and might be
> > added after probe.
>
> Correct, it can't tell them apart. Which is why it tries its best to
> enforce them, get most of them ordered properly and then gives up
> enforcing the rest after deferred_probe_timeout= expires. There's a
> bit more nuance than what I explained here (explained in earlier
> commit texts, LPC talks), but that's the gist of it. That's what's
> going on in this case Sascha is pointing out.z
>
> > We need a strong devlink for the first category
> > and maybe a weak devlink for the latter category.
> >
> > I don't know if this is a generic hardware property for all operating
> > systems so it could be a DT property such as dma-weak-dependency?
> >
> > Or maybe compromize and add a linux,dma-weak-dependency;
> > property?
>
> The linux,dma-weak-dependency might be an option, but then if the
> kernel version changes and we want to enforce it because we now have a
> dma driver (not related to Shasha's example) support, then the
> fw_devlink still can't enforce it because of that property. But maybe
> that's okay? The consumer can try to use dma and defer probe if it
> fails?
>
> Another option is to mark console devices in DT with some property and
> we can give special treatment for those without waiting for
> deferred_probe_timeout= to expire.

Heh, looks like there's already a property for that: stdout-path.

Let me send a series that'll use that to give special treatment to
console devices.

-Saravana