linux-kernel - Re: Regression in 5.4 kernel on 32-bit Radeon IBM T40

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Date:   Thu, 9 Jan 2020 15:14:36 +0100
From:   Christoph Hellwig <hch@....de>
To:     Woody Suwalski <terraluna977@...il.com>
Cc:     hch@....de, DRI mailing list <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Christian König <christian.koenig@....com>,
        "Deucher, Alexander" <alexander.deucher@....com>,
        Pavel Machek <pavel@....cz>
Subject: Re: Regression in 5.4 kernel on 32-bit Radeon IBM T40

Hi Woody,

sorry for the late reply, I've been off to a vacation over the holidays.

On Sat, Dec 14, 2019 at 10:17:15PM -0500, Woody Suwalski wrote:
> Regression in 5.4 kernel on 32-bit Radeon IBM T40
> triggered by
> commit 33b3ad3788aba846fc8b9a065fe2685a0b64f713
> Author: Christoph Hellwig <hch@....de>
> Date:   Thu Aug 15 09:27:00 2019 +0200
>
> Howdy,
> The above patch has triggered a display problem on IBM Thinkpad T40, where 
> the screen is covered with a lots of random short black horizontal lines, 
> or distorted letters in X terms.
>
> The culprit seems to be that the dma_get_required_mask() is returning a 
> value 0x3fffffff
> which is smaller than dma_get_mask()0xffffffff.That results in 
> dma_addressing_limited()==0 in ttm_bo_device(), and using 40-bits dma 
> instead of 32-bits.

Which is the intended behavior assuming your system has 1GB of memory.
Does it?

> If I hardcode "1" as the last parameter to ttm_bo_device_init() in place of 
> a call to dma_addressing_limited(),the problem goes away.

I'll need some help from the drm / radeon / TTM maintainers if there are
any other side effects from not passing the need_dma32 paramters.
Obviously if the device doesn't have more than 32-bits worth of dram and
no DMA offset we can't feed unaddressable memory to the device.
Unfortunately I have a very hard time following the implementation of
the TTM pool if it does anything else in this case.