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Subject: Re: Regression in 5.4 kernel on 32-bit Radeon IBM T40
To:     Christoph Hellwig <hch@lst.de>,
        Woody Suwalski <terraluna977@gmail.com>
Cc:     DRI mailing list <dri-devel@lists.freedesktop.org>,
        LKML <linux-kernel@vger.kernel.org>,
        "Deucher, Alexander" <alexander.deucher@amd.com>,
        Pavel Machek <pavel@ucw.cz>
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From:   =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>
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Date:   Thu, 9 Jan 2020 16:12:26 +0100
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Hi Christoph,

Am 09.01.20 um 15:14 schrieb Christoph Hellwig:
> 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@lst.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?

Assuming the system doesn't have the 1GB split up somehow crazy over the 
address space that should indeed work as intended.

>
>> 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.

The only other thing which comes to mind is using huge pages. Can you 
try a kernel with CONFIG_TRANSPARENT_HUGEPAGE disabled?

Thanks,
Christian.