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Date:   Fri, 11 May 2018 09:55:04 +1000 (AEST)
From:   Finn Thain <fthain@telegraphics.com.au>
To:     Michael Schmitz <schmitzmic@gmail.com>
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>
Subject: Re: [PATCH net] macmace: Set platform device coherent_dma_mask
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On Fri, 11 May 2018, Michael Schmitz wrote:

> > > Perhaps you can add a new helper 
> > > (platform_device_register_simple_dma()?) that takes the DMA mask, 
> > > too?
...
> >
> > So far, it looks like macmace and macsonic would be the only callers 
> > of this new API call.
> >
> > What's worse, if you do pass a dma_mask in struct 
> > platform_device_info, you end up with this problem in 
> > platform_device_register_full():
> >
> >         if (pdevinfo->dma_mask) {
> >                 /*
> >                  * This memory isn't freed when the device is put,
> >                  * I don't have a nice idea for that though.  Conceptually
> >                  * dma_mask in struct device should not be a pointer.
> >                  * See http://thread.gmane.org/gmane.linux.kernel.pci/9081
> >                  */
> >                 pdev->dev.dma_mask =
> >                         kmalloc(sizeof(*pdev->dev.dma_mask), GFP_KERNEL);
> 
> Maybe platform_device_register_full() should rather check whether 
> dev.coherent_dma_mask is set, and make dev.dma_mask point to that? This 
> is how we solved the warning issue for the Zorro bus devices... 
> (8614f1b58bd0e920a5859464a500b93152c5f8b1)
> 

The claim in the comment above that a pointer is the wrong solution 
suggests that your proposal won't get far. Also, your proposal doesn't 
address the other issues I raised: a new 
platform_device_register_simple_dma() API would only have two callers, and 
the dma mask setup for device-tree probed platform devices is apparently a 
work-in-progress (which I don't want to churn up).

> > > With people setting the mask to kill the WARNING splat, this may 
> > > become more common.
> >
> > Since the commit which introduced the WARNING, only commits f61e64310b75
> > ("m68k: set dma and coherent masks for platform FEC ethernets") and
> > 7bcfab202ca7 ("powerpc/macio: set a proper dma_coherent_mask") seem to be
> > aimed at squelching that WARNING.
> >
> > (Am I missing any others?)
> 
> Zorro devices :-)

Right, I should add commit 55496d3fe2ac ("zorro: Set up z->dev.dma_mask 
for the DMA API") to that list.

> 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.

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