On Wed, Dec 4, 2013 at 6:56 PM, Alex Deucher <alexdeucher at gmail.com> wrote:
> On Wed, Dec 4, 2013 at 5:16 PM, Kleber Sacilotto de Souza
> <klebers at linux.vnet.ibm.com> wrote:
>> On 11/25/2013 10:11 PM, Kleber Sacilotto de Souza wrote:
>>>
>>> On 11/24/2013 09:15 PM, Benjamin Herrenschmidt wrote:
>>>>
>>>> On Fri, 2013-11-08 at 11:43 -0200, Kleber Sacilotto de Souza wrote:
>>>>>
>>>>> On 11/07/2013 08:29 PM, Benjamin Herrenschmidt wrote:
>>>>>>
>>>>>> On Mon, 2013-06-17 at 18:57 -0400, Alex Deucher wrote:
>>>>>>
>>>>>>> Weird. I wonder if there is an issue with cache snoops on PPC. We
>>>>>>> currently use the gart in cached mode (GPU snoops CPU cache) with
>>>>>>> cached pages. I wonder if we need to use uncached pages on PPC.
>>>>>>
>>>>>> There is no such issue and no known bugs with DMA writes on those
>>>>>> PCIe host bridges (and they do get hammered pretty bad here).
>>>>>>
>>>>>> This needs further investigation by the lab/hw guys to find out what's
>>>>>> actually happening on the bus and the host bridge.
>>>>>>
>>>>>> Thadeu, Kleber: Jerome suggested writing a test case in userspace that
>>>>>> continuously writes to a spare scratch register (thus triggering the
>>>>>> corresponding writeback DMA) and checks the memory location to compare
>>>>>> the writeback value (using a debugfs file for example, or mmap).
>>
>>
>> I was not able to reproduce the issue with this method, even after a weekend
>> run.
>>
>> However, doing some more investigation it seems the problem is here, where
>> we read the ring rptr:
>>
>> u32 radeon_ring_generic_get_rptr(struct radeon_device *rdev,
>> struct radeon_ring *ring)
>> {
>> u32 rptr;
>>
>> if (rdev->wb.enabled)
>> rptr = le32_to_cpu(rdev->wb.wb[ring->rptr_offs/4]);
>> else
>> rptr = RREG32(ring->rptr_reg);
>>
>> return rptr;
>> }
>>
>> I realized that the DMA'ed rptr value has always the opposite byte order
>> from the MMIO value. Since RREG32 already returns the register value on the
>> CPU byte order, it seems we don't need to byte-swap the DMA'ed value. If I
>> remove the le32_to_cpu() call and use the DMA'ed value directly, I don't get
>> the IB scheduling failures and piglit results are the same as with writeback
>> disabled.
>>
>> Is the adapter chipset swapping the bytes before doing the DMA to a
>> big-endian host?
>
> Setting CP_RB_CNTL.BUF_SWAP causes the CP to use the selected byte
> swapping for just about everything accessed by the CP (rptr writeback,
> indirect buffers, etc.). Looks like the DMA ring supports and enables
> rptr writeback as well (DMA_RB_CNTL.DMA_RPTR_WRITEBACK_SWAP_ENABLE) so
> I think we can drop the swapping of the rptr writeback.
>
Obvious patch attached.
Alex
> Alex
>
>>
>>
>>
>> --
>> Kleber Sacilotto de Souza
>> IBM Linux Technology Center
>>
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