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Message-ID: <alpine.LNX.2.21.1805291047230.13@nippy.intranet>
Date: Tue, 29 May 2018 12:15:05 +1000 (AEST)
From: Finn Thain <fthain@...egraphics.com.au>
To: Geert Uytterhoeven <geert@...ux-m68k.org>
cc: Christoph Hellwig <hch@....de>,
Michael Schmitz <schmitzmic@...il.com>,
Guenter Roeck <linux@...ck-us.net>,
Joshua Thompson <funaho@...ai.org>,
Greg Ungerer <gerg@...ux-m68k.org>,
linux-m68k <linux-m68k@...ts.linux-m68k.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] m68k: set dma and coherent masks for Macintosh SONIC
based ethernet
On Mon, 28 May 2018, Geert Uytterhoeven wrote:
>
> Do we have a consensus on the way forward?
My prefered solution remains the two driver patches that I originally
submitted, which you objected to:
https://lkml.org/lkml/2018/5/3/10
https://lkml.org/lkml/2018/5/3/9
So there is no consensus yet.
To my mind, the existing code is pretty clear: drivers should set the
(default) mask. See also,
$ egrep -r -A1 "coherent_dma_mask.*expected" */
drivers/of/device.c: * Set default coherent_dma_mask to 32 bit. Drivers are expected to
drivers/of/device.c- * setup the correct supported mask.
--
drivers/acpi/arm64/iort.c: * Set default coherent_dma_mask to 32 bit. Drivers are expected to
drivers/acpi/arm64/iort.c- * setup the correct supported mask.
$
And drivers/usb/dwc2/platform.c:
/*
* Use reasonable defaults so platforms don't have to provide these.
*/
if (!dev->dev.dma_mask)
dev->dev.dma_mask = &dev->dev.coherent_dma_mask;
retval = dma_set_coherent_mask(&dev->dev, DMA_BIT_MASK(32));
And FWIW,
$ egrep -wlr "dma_set_mask_and_coherent|dma_set_coherent_mask|dma_coerce_mask_and_coherent" drivers/ | wc -l
196
I suppose that a driver should avoid lengthening an existing device mask.
Since an arch gets to apply limits in the dma ops it implements, why would
arch code also have to set a limit in the form of default platform device
masks? Powerpc seems to be the only arch that does this.
The same line of reasoning suggests that the problematic WARN_ON should
not appear in include/linux/ in the first place. If it is needed by
certain architectures, it should be in arch/x.
I would send a patch to revert commit 205e1b7f51e4 ("dma-mapping: warn
when there is no coherent_dma_mask") if I thought that arch code was not
somehow relying on it. But I'll leave that up to Chrisoph.
--
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