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

[Andy Shevchenko <andy.shevchenko@xxxxxxxxx>]

On Wed, Jun 22, 2022 at 10:44 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.
> >
> > 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.

In 8250 we force disable DMA and PM on kernel consoles, because it's
so-o PITA and has a lot of corner cases we may never chase down.

089b6d365491 serial: 8250_port: Disable DMA operations for kernel console
bedb404e91bb serial: 8250_port: Don't use power management for kernel console


> 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. 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?


-- 
With Best Regards,
Andy Shevchenko