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

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Message-ID: <bf7667f7-a11f-87ac-3aca-130cca86d328@gmail.com>
Date:   Fri, 4 May 2018 20:16:58 +1200
From:   Michael Schmitz <schmitzmic@...il.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Christoph Hellwig <hch@....de>,
        Finn Thain <fthain@...egraphics.com.au>,
        "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>
Subject: Re: [PATCH net] macmace: Set platform device coherent_dma_mask

Hi Geert,

Am 04.05.2018 um 19:24 schrieb Geert Uytterhoeven:
> Hi Michael,
> 
>>> Yes, that would be useful.  The other assumption could be that
>>> platform devices always allow an all-0xff dma mask.
>>
>> That's not always true (Atari NCR5380 SCSI and floppy would use a 24
>> bit DMA mask). We use bounce buffers allocated from a dedicated lowmem
>> pool there currently, and for all I know don't use the DMA API yet.
>>
>> I bet that is a rare exception though. Setting the default DMA mask
>> for platform devices to all-0xff and letting the few odd drivers force
>> a different setting seems the best way forward.
> 
> I'd say that's usually a property of the platform, not of the device?

Right - I was thinking 'm68k' as platform, not a particular machine like
Mac or Falcon (the 24 bit mask only applies to that particular model
anyway).

> So IMHO it belongs in the platform code, not in the device driver code.

OK - let's have a default mask of 64 bit, and allow machine specific
platform_init() to override using a new helper function.

Cheers,

	Michael

> Gr{oetje,eeting}s,
> 
>                         Geert
>