netdev - Re: [PATCH v2 7/9] driver core: Set fw

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Date:   Wed, 22 Jun 2022 12:52:02 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     Sascha Hauer <sha@...gutronix.de>,
        Saravana Kannan <saravanak@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Kevin Hilman <khilman@...nel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Andrew Lunn <andrew@...n.ch>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Russell King <linux@...linux.org.uk>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Android Kernel Team <kernel-team@...roid.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        "list@....net:IOMMU DRIVERS" <iommu@...ts.linux-foundation.org>,
        netdev <netdev@...r.kernel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Sascha Hauer <kernel@...gutronix.de>
Subject: Re: [PATCH v2 7/9] driver core: Set fw_devlink.strict=1 by default

On Wed, Jun 22, 2022 at 10:44 AM Linus Walleij <linus.walleij@...aro.org> wrote:
> On Wed, Jun 22, 2022 at 9:48 AM Sascha Hauer <sha@...gutronix.de> 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