linux-kernel - Re: [BISECTED REGRESSION] b43legacy broken on G4 PowerBook

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Message-ID: <35b3f09b-b371-e2cc-4436-120c67e2f1fb@lwfinger.net>
Date:   Wed, 5 Jun 2019 22:06:18 -0500
From:   Larry Finger <Larry.Finger@...inger.net>
To:     Aaro Koskinen <aaro.koskinen@....fi>,
        Christoph Hellwig <hch@....de>,
        Christian Zigotzky <chzigotzky@...osoft.de>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     linux-kernel@...r.kernel.org, linux-wireless@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [BISECTED REGRESSION] b43legacy broken on G4 PowerBook

On 6/5/19 5:50 PM, Aaro Koskinen wrote:
> Hi,
> 
> When upgrading from v5.0 -> v5.1 on G4 PowerBook, I noticed WLAN does
> not work anymore:
> 
> [   42.004303] b43legacy-phy0: Loading firmware version 0x127, patch level 14 (2005-04-18 02:36:27)
> [   42.184837] b43legacy-phy0 debug: Chip initialized
> [   42.184873] b43legacy-phy0 ERROR: The machine/kernel does not support the required 30-bit DMA mask
> 
> The same happens with the current mainline.
> 
> Bisected to:
> 
> 	commit 65a21b71f948406201e4f62e41f06513350ca390
> 	Author: Christoph Hellwig <hch@....de>
> 	Date:   Wed Feb 13 08:01:26 2019 +0100
> 
> 	    powerpc/dma: remove dma_nommu_dma_supported
> 
> 	    This function is largely identical to the generic version used
> 	    everywhere else.  Replace it with the generic version.
> 
> 	    Signed-off-by: Christoph Hellwig <hch@....de>
> 	    Tested-by: Christian Zigotzky <chzigotzky@...osoft.de>
> 	    Signed-off-by: Michael Ellerman <mpe@...erman.id.au>

Aaro,

First of all, you have my sympathy for the laborious bisection on a PowerBook 
G4. I have done several myself. Thank you.

I confirm your results.

The ppc code has a maximum DMA size of 31 bits, thus a 32-bit request will fail. 
Why the 30-bit fallback fails in b43legacy fails while it works in b43 is a mystery.

Although dma_nommu_dma_supported() may be "largely identical" to 
dma_direct_supported(), they obviously differ. Routine dma_nommu_dma_supported() 
returns 1 for 32-bit systems, but I do not know what dma_direct_supported() returns.

I am trying to find a patch.

Larry