netdev - Re: [PATCH net] macmace: Set platform device coherent_dma

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Date:   Fri, 11 May 2018 15:28:58 +1000 (AEST)
From:   Finn Thain <fthain@...egraphics.com.au>
To:     Michael Schmitz <schmitzmic@...il.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>
cc:     "David S. Miller" <davem@...emloft.net>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        netdev <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH net] macmace: Set platform device coherent_dma_mask

On Fri, 11 May 2018, Michael Schmitz wrote:

> 
> I'm afraid using platform_device_register() (which you already use for 
> the SCC devices) is the only option handling this on a per-device basis 
> without touching platform core code, while at the same time keeping the 
> DMA mask setup out of device drivers

I don't think that will fly. If you call platform_device_register() and 
follow that with a dma mask assignment, you could race with the bus 
matching and driver probe, and we are back to the same WARNING message.

If you want to use platform_device_register(), you'd have to implement 
arch_setup_pdev_archdata() and use that to set up the dma mask.

> (I can see Geert's point there - device driver code might be shared 
> across implementations of the device on platforms with different DMA 
> mask requirements,, something the driver can't be expected to know 
> about).

As I said, these drivers might be expected to be portable between Macs and 
early PowerMacs, but the same dma mask would apply AFAIK.

If a platform driver isn't expected to be portable, I think either method 
is reasonable: arch_setup_pdev_archdata() or the method in the patch.

Anyway, there is this in arch/powerpc/kernel/setup-common.c:

void arch_setup_pdev_archdata(struct platform_device *pdev)
{
        pdev->archdata.dma_mask = DMA_BIT_MASK(32);
        pdev->dev.dma_mask = &pdev->archdata.dma_mask;
	...

I'm inclined to propose something similar for m68k. That should fix the 
problem, since arch_setup_pdev_archdata() is already in the call chain:

	platform_device_register_simple()
		platform_device_register_resndata()
			platform_device_register_full()
				platform_device_alloc()
					arch_setup_pdev_archdata()

Thoughts? Will this have nasty side effects for m68k platforms that use 
smaller dma masks?

-- 

> 
> Cheers,
> 
> 	Michael
>