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

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Message-ID: <CAOmrzk+y5pmm2anaw15pch--y_gqoUO4NQemSbSanAXpEnttkg@mail.gmail.com>
Date:   Fri, 4 May 2018 08:24:38 +1200
From:   Michael Schmitz <schmitzmic@...il.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        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 Christoph,

On Thu, May 3, 2018 at 8:51 PM, Christoph Hellwig <hch@....de> wrote:
> On Thu, May 03, 2018 at 10:46:56AM +0200, Geert Uytterhoeven wrote:
>> Perhaps you can add a new helper (platform_device_register_simple_dma()?)
>> that takes the DMA mask, too?
>> With people setting the mask to kill the WARNING splat, this may become
>> more common.
>>
>> struct platform_device_info already has a dma_mask field, but
>> platform_device_register_resndata() explicitly sets it to zero.
>
> 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.

Cheers,

  Michael



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