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Subject: Re: [PATCH net] macmace: Set platform device coherent_dma_mask
To:     Finn Thain <fthain@telegraphics.com.au>
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Cc:     Geert Uytterhoeven <geert@linux-m68k.org>,
        "David S. Miller" <davem@davemloft.net>,
        linux-m68k <linux-m68k@lists.linux-m68k.org>,
        netdev <netdev@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Christoph Hellwig <hch@lst.de>
From:   Michael Schmitz <schmitzmic@gmail.com>
Message-ID: <23ed7ff8-c1d7-a069-3058-3944c7586116@gmail.com>
Date:   Sat, 12 May 2018 10:02:03 +1200
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Hi Finn,

Am 11.05.2018 um 22:06 schrieb Finn Thain:
>> You would have to be careful not to overwrite a pdev->dev.dma_mask and 
>> pdev->dev.dma_coherent_mask that might have been set in a platform 
>> device passed via platform_device_register here. Coldfire is the only 
>> m68k platform currently using that, but there might be others in future.
>>
> 
> That Coldfire patch could be reverted if this is a better solution.

True, but there might be other uses for deviating from a platform
default (I'm thinking of Atari SCSI and floppy drivers here). But we
could chose the correct mask to set in arch_setup_pdev_archdata()
instead, as it's a platform property not a driver property in that case.

>> ... But I don't think there are smaller DMA masks used by m68k drivers 
>> that use the platform device mechanism at present. I've only looked at 
>> arch/m68k though.
> 
> So we're back at the same problem that Geert's suggestion also raised: how 
> to identify potentially affected platform devices and drivers?
> 
> Maybe we can take a leaf out of Christoph's book, and leave a noisy 
> WARNING splat in the log.
> 
> void arch_setup_pdev_archdata(struct platform_device *pdev)
> {
>         WARN_ON_ONCE(pdev->dev.coherent_dma_mask != DMA_MASK_NONE ||
>                      pdev->dev.dma_mask != NULL);

I'd suggest using WARN_ON() so we catch all uses on a particular platform.

I initially thought it necessary to warn on unset mask here, but I see
that would throw up a lot of redundant false positives.

Cheers,

	Michael