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Subject: Re: Regression in 5.4 kernel on 32-bit Radeon IBM T40
From:   Woody Suwalski <terraluna977@gmail.com>
To:     =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>,
        Christoph Hellwig <hch@lst.de>
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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Date:   Thu, 9 Jan 2020 21:40:50 -0500
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Woody Suwalski wrote:
> Christian König wrote:
>> 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.
>
> Happy New Year :-)
>
> Yes, the box has 1G of RAM, and unfortunately nope, 
> TRANSPARENT_HUGEPAGE is not on. I am attaching the .config, maybe you 
> can find some insanity there... Also - for reference - a minimalistic 
> patch fixing symptoms (but not addressing the root cause  :-( )
>
> I can try to rebuild the kernel with HIGHMEM off, although I am not 
> optimistic it will change anything. But at least it should simplify 
> the 1G split...
>
> So if you have any other ideas - pls let me know..
>
> Thanks, Woody
>
Interesting. Rebuilding the kernel with HIMEM disabled actually solves 
the display problem. The debug lines show exactly same values for 
dma_get_required_mask() and dma_get_mask(), yet now it works OK... So 
what has solved it???

Woody