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

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Date:   Wed, 22 Jun 2022 10:44:01 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Sascha Hauer <sha@...gutronix.de>
Cc:     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>, kernel-team@...roid.com,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        iommu@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-gpio@...r.kernel.org, kernel@...gutronix.de
Subject: Re: [PATCH v2 7/9] driver core: Set fw_devlink.strict=1 by default

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.

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?

Yours,
Linus Walleij