linux-kernel - Re: [PATCH v3 02/16] mfd: mfd-core: Don't overwrite the dma

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Message-ID: <3cd3705a-4f48-6a46-e869-3ee11dc17323@arm.com>
Date:   Tue, 28 Apr 2020 14:06:20 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Michael Walle <michael@...le.cc>
Cc:     linux-gpio@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-hwmon@...r.kernel.org,
        linux-pwm@...r.kernel.org, linux-watchdog@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Linus Walleij <linus.walleij@...aro.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Rob Herring <robh+dt@...nel.org>,
        Jean Delvare <jdelvare@...e.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Lee Jones <lee.jones@...aro.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Uwe Kleine-König <u.kleine-koenig@...gutronix.de>,
        Wim Van Sebroeck <wim@...ux-watchdog.org>,
        Shawn Guo <shawnguo@...nel.org>, Li Yang <leoyang.li@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jason Cooper <jason@...edaemon.net>,
        Marc Zyngier <maz@...nel.org>, Mark Brown <broonie@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v3 02/16] mfd: mfd-core: Don't overwrite the dma_mask of
 the child device

On 2020-04-28 1:45 pm, Andy Shevchenko wrote:
> On Thu, Apr 23, 2020 at 07:45:29PM +0200, Michael Walle wrote:
>> Commit cdfee5623290 ("driver core: initialize a default DMA mask for
>> platform device") initialize the DMA of a platform device. But if the
>> parent doesn't have a dma_mask set, for example if it's an I2C device,
>> the dma_mask of the child platform device will be set to zero again.
>> Which leads to many "DMA mask not set" warnings, if the MFD cell has the
>> of_compatible property set.
> 
> I'm wondering why parent doesn't have it.

Because the parent isn't on a DMA-capable bus, and thus really shouldn't 
have a valid DMA configuration ever.

> I remember we have explicit patches in the past for buses such as PCI and AMBA
> to set default DMA mask for all physical devices on the respective bus, of
> course they can individually override it later.
> 
> So, this seems to me a paper over the real issue (absence of default DMA mask
> where it's needed) and devices should explicitly define it if they disagree
> with default.
> 
> If I'm wrong, you really need elaborate commit message much better.

The problem here is that MFD children are created as platform devices 
(regardless of what their parent is) and assigned an of_node, at which 
point they look pretty much indistinguishable from SoC devices created 
by the of_platform code, that *do* have to be assumed to be DMA-capable 
to prevent ~90% of existing devicetrees from breaking.

Of course the real fundamental issue is the platform bus itself, but 
it's way too late to fix that :(

Robin.