Re: [PATCH] drm/radeon: Disable writeback by default on ppc

[Kleber Sacilotto de Souza <klebers@linux.vnet.ibm.com>]

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?


-- 
Kleber Sacilotto de Souza
IBM Linux Technology Center