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

From: Andy Shevchenko <andy.shevchenko@gmail.com>
To: Linus Walleij <linus.walleij@linaro.org>
Cc: Sascha Hauer <sha@pengutronix.de>,
	Saravana Kannan <saravanak@google.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	Kevin Hilman <khilman@kernel.org>,
	Ulf Hansson <ulf.hansson@linaro.org>,
	Len Brown <len.brown@intel.com>, Pavel Machek <pavel@ucw.cz>,
	Joerg Roedel <joro@8bytes.org>, Will Deacon <will@kernel.org>,
	Andrew Lunn <andrew@lunn.ch>,
	Heiner Kallweit <hkallweit1@gmail.com>,
	Russell King <linux@armlinux.org.uk>,
	"David S. Miller" <davem@davemloft.net>,
	Eric Dumazet <edumazet@google.com>,
	Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
	Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
	David Ahern <dsahern@kernel.org>,
	Android Kernel Team <kernel-team@android.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Linux PM <linux-pm@vger.kernel.org>,
	"list@263.net:IOMMU DRIVERS" <iommu@lists.linux-foundation.org>,
	netdev <netdev@vger.kernel.org>,
	"open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>,
	Sascha Hauer <kernel@pengutronix.de>
Subject: Re: [PATCH v2 7/9] driver core: Set fw_devlink.strict=1 by default
Date: Wed, 22 Jun 2022 12:52:02 +0200	[thread overview]
Message-ID: <CAHp75VdqjCoWAHV4AyYrju0o8buREA8pM5wyf8TD=rCMTs-tEA@mail.gmail.com> (raw)
In-Reply-To: <CACRpkdYe=u9Ozj_dtLVr6GSau8yS5H7LnBNNrQHki1CJ1zST0A@mail.gmail.com>

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