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

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Message-ID: <0b13a08d-ba2d-b9dc-4fd9-1f6bad5cd1ee@gmail.com>
Date:   Fri, 11 May 2018 16:18:01 +1200
From:   Michael Schmitz <schmitzmic@...il.com>
To:     Finn Thain <fthain@...egraphics.com.au>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        "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

Hi Finn,

Am 11.05.2018 um 15:28 schrieb Finn Thain:
> On Fri, 11 May 2018, Michael Schmitz wrote:
> 
>>>> Which begs the question: why can' you set up all Nubus bus devices' 
>>>> DMA masks in nubus_device_register(), or nubus_add_board()?
>>>
>>> I am expecting to see the same WARNING from the nubus sonic driver but 
>>> it hasn't happened yet, so I don't have a patch for it yet. In 
>>> anycase, the nubus fix would be a lot like the zorro bus fix, so I 
>>> don't see a problem.
>>
>> That's odd. But what I meant to say is that by setting up 
>> dma_coherent_mask in nubus_add_board(), and pointing dma_mask to that, 
>> ypu won't need any patches to Nubus device drivers.
> 
> Right. I think I've already acknowledged that. But it's off-topic, because 
> the patches under review are for platform drivers. Those patches fix an 
> actual bug that I've observed. Whereas, the nubus driver dma mask issue 
> that you raised is purely theoretical at this stage.

I had lost track of the fact that macsonic can be probed as either Nubus
or platform device. Sorry for the noise.

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 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).

Cheers,

	Michael