Long radeon stalls on recent kernels

[Alex Deucher]

On Tue, Dec 9, 2014 at 4:39 PM, Andy Lutomirski <luto at amacapital.net> wrote:
> On Tue, Dec 9, 2014 at 8:06 AM, Andy Lutomirski <luto at amacapital.net> wrote:
>> On Tue, Dec 9, 2014 at 1:18 AM, Michel Dänzer <michel at daenzer.net> wrote:
>>> On 09.12.2014 09:24, Andy Lutomirski wrote:
>>>>
>>>> The relevant line from latencytop seems to be:
>>>>
>>>> 154 20441402 489139 radeon_fence_default_wait [radeon]
>>>> fence_wait_timeout ttm_bo_wait [ttm] ttm_bo_move_accel_cleanup [ttm]
>>>> radeon_move_blit.isra.12 [radeon] radeon_bo_move [radeon]
>>>> ttm_bo_handle_move_mem [ttm] ttm_bo_evict [ttm] ttm_mem_evict_first
>>>> [ttm] ttm_bo_mem_space [ttm] ttm_bo_validate [ttm]
>>>> radeon_bo_fault_reserve_notify [radeon]
>>>
>>> Which process is this?
>>
>> Xorg
>>
>>>
>>> Looks like CPU access to a BO in VRAM, but the BO is located outside of
>>> the CPU visible area of VRAM, so it has to be moved into the CPU visible
>>> area first.
>>>
>>> Which version of Mesa are you using?
>>>
>>
>> mesa-dri-drivers-10.3.3-1.20141110.fc20.x86_64
>>
>> I'm planning on upgrading to Fedora 21 fairly soon.
>
> Upgrading to mesa-dri-drivers-10.3.3-1.20141110.fc21.x86_64 seems to
> have helped enough that my usual test (open a couple of Firefox tabs
> with graphics in them) doesn't hang anymore.
>
> This card still isn't *fast*.  Is there some way I can check that I'm
> actually using all 16 PCIe lanes?  In my tinkering w/ power management
> settings, I got some odd logs suggesting that only one lane was in
> use.
>

You should be using all the lanes available.  The main issue with that
card is vram memory bandwidth.  Those chips have a single channel
memory interface and most OEMs populate them with DDR3 memory rather
than GDDR5.

from your log:
[    3.079577] [drm] RAM width 64bits DDR
...
[    3.082589] [drm] enabling PCIE gen 2 link speeds, disable with
radeon.pcie_gen2=0

Alex