linux-kernel - Re: [PATCH] drm/tests/drm_buddy: avoid 64-bit calculation

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Message-ID: <a293460f-6a40-427f-b5d2-2e701d1af229@amd.com>
Date: Mon, 19 Feb 2024 12:41:20 +0100
From: Christian König <christian.koenig@....com>
To: Arnd Bergmann <arnd@...db.de>, Randy Dunlap <rdunlap@...radead.org>,
 Arnd Bergmann <arnd@...nel.org>,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>
Cc: Dave Airlie <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>,
 Arunpravin Paneer Selvam <Arunpravin.PaneerSelvam@....com>,
 David Gow <davidgow@...gle.com>, Maíra Canal
 <mcanal@...lia.com>, Matthew Auld <matthew.auld@...el.com>,
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/tests/drm_buddy: avoid 64-bit calculation

Am 19.02.24 um 12:29 schrieb Arnd Bergmann:
> On Mon, Feb 19, 2024, at 12:22, Christian König wrote:
>> Am 17.02.24 um 02:31 schrieb Randy Dunlap:
>>> On 2/16/24 12:24, Arnd Bergmann wrote:
>>>> From: Arnd Bergmann <arnd@...db.de>
>>>>
>>>> The newly added drm_test_buddy_alloc_contiguous() test fails to link on
>>>> 32-bit targets because of inadvertent 64-bit calculations:
>>>>
>>>> ERROR: modpost: "__aeabi_uldivmod" [drivers/gpu/drm/tests/drm_buddy_test.ko] undefined!
>>>> ERROR: modpost: "__aeabi_ldivmod" [drivers/gpu/drm/tests/drm_buddy_test.ko] undefined!
>>>>
>>>> >From what I can tell, the numbers cannot possibly overflow a 32-bit size,
>>>> so use different types for these.
>>>>
>>>> I noticed that the function has another possible flaw in that is mixes
>>>> what it calls pages with 4KB units. This is a big confusing at best,
>>>> or possibly broken when built on machines with larger pages.
>>>>
>>>> Fixes: a64056bb5a32 ("drm/tests/drm_buddy: add alloc_contiguous test")
>>>> Signed-off-by: Arnd Bergmann <arnd@...db.de>
>>> Tested-by: Randy Dunlap <rdunlap@...radead.org>
>> I've just pushed a similar patch Mathew came up a bit earlier to
>> drm-misc-fixes.
>>
>> Sorry for the noise, I have to catch up on picking up patches for
>> misc-fixes and misc-next.
> Ok, thanks.
>
> Have you looked at how this code works for larger values of PAGE_SIZE?
> Is there any need to change other things or will this work with the
> hardcoded 4KB chunks?

I haven't looked into the details, but I've pointed out before that 
using PAGE_SIZE in the buddy or its test cases would be incorrect.

Background is that the buddy allocator is for devices and those work 
independent of the CPU PAGE_SIZE. So it can be that on a CPU with 64k 
pages the buddy still needs to work with 4k.

Could be that this is work, but could as well be that this is completely 
broken. Arun and Mathew needs to answer this, I haven't tested it nor 
reviewed the code.

Regards,
Christian.

>
>       Arnd