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Subject: Re: [BISECTED REGRESSION] b43legacy broken on G4 PowerBook
To:     Christoph Hellwig <hch@lst.de>
Cc:     Aaro Koskinen <aaro.koskinen@iki.fi>,
        Christian Zigotzky <chzigotzky@xenosoft.de>,
        Michael Ellerman <mpe@ellerman.id.au>,
        linux-kernel@vger.kernel.org, linux-wireless@vger.kernel.org,
        linuxppc-dev@lists.ozlabs.org
References: <20190605225059.GA9953@darkstar.musicnaut.iki.fi>
 <73da300c-871c-77ac-8a3a-deac226743ef@lwfinger.net>
 <20190607172902.GA8183@lst.de>
From:   Larry Finger <Larry.Finger@lwfinger.net>
Message-ID: <30000803-3772-3edf-f4a9-55122d504f3f@lwfinger.net>
Date:   Sat, 8 Jun 2019 16:52:24 -0500
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On 6/7/19 12:29 PM, Christoph Hellwig wrote:
> I don't think we should work around this in the driver, we need to fix
> it in the core.  I'm curious why my previous patch didn't work.  Can
> you throw in a few printks what failed?  I.e. did dma_direct_supported
> return false?  Did the actual allocation fail?

Routine dma_direct_supported() returns true.

The failure is in routine dma_set_mask() in the following if test:

         if (!dev->dma_mask || !dma_supported(dev, mask))
                 return -EIO;

For b43legacy, dev->dma_mask is 0xc265684800000000.
     dma_supported(dev, mask) is 0xc08b000000000000, mask is 0x3fffffff, and the 
routine returns -EIO.

For b43,       dev->dma_mask is 0xc265684800000001,
     dma_supported(dev, mask) is 0xc08b000000000000, mask is 0x77777777, and the 
routine returns 0.

Thus far I have not found what sets the low-order bit of dev->dma_mask. 
Suggestions are welcome.

These tests have all been with your patch that sets ARCH_ZONE_DMA_BITS to 30.

Larry