Re: [PATCH] drm/radeon: make 64bit fences more robust

["Christian König" <deathsimple@vodafone.de>]

On 10.09.2012 13:12, Michel Dänzer wrote:
> On Mon, 2012-09-10 at 11:13 +0200, Christian König wrote:
>> Only increase the higher 32bits if we really detect a wrap around.
>>
>> Fixes:
>> https://bugs.freedesktop.org/show_bug.cgi?id=54129
>> https://bugs.freedesktop.org/show_bug.cgi?id=54662
>>
>> Possible fixes:
>> https://bugzilla.redhat.com/show_bug.cgi?id=846505
>> https://bugzilla.redhat.com/show_bug.cgi?id=845639
>>
>> Signed-off-by: Christian König <deathsimple@vodafone.de>
>> Cc: stable@vger.kernel.org
>> ---
>>   drivers/gpu/drm/radeon/radeon_fence.c |    6 +++---
>>   1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/radeon/radeon_fence.c b/drivers/gpu/drm/radeon/radeon_fence.c
>> index 7b737b9..4781e13 100644
>> --- a/drivers/gpu/drm/radeon/radeon_fence.c
>> +++ b/drivers/gpu/drm/radeon/radeon_fence.c
>> @@ -160,7 +160,7 @@ void radeon_fence_process(struct radeon_device *rdev, int ring)
>>   	do {
>>   		seq = radeon_fence_read(rdev, ring);
>>   		seq |= last_seq & 0xffffffff00000000LL;
>> -		if (seq < last_seq) {
>> +		if (seq < (last_seq - 0x80000000LL)) {
>>   			seq += 0x100000000LL;
>>   		}
> Can you provide a bit more explanation for this change? In particular,
> how could the code previously detect a wraparound when there was none,
> and why is this the proper fix?

Honestly I also don't really understand how this bug happened in the 
first place.

We extend the 32bit fences supported by hardware by testing if a 
previously read fence value is smaller than the value we read now:

>		if (seq < last_seq) {

But the problem seems to be that on some systems we do get fence values 
that are decreasing, e.g. instead of 5, 6, 7, 8 we get 5, 7, 6, 8 (or 
maybe 5, 6, 0, 7, 8 because somebody accidentally overwrites the fence 
value).

It might be related to a hardware bug, or the algorithm is flawed in a 
way I currently don't see. Anyway the old code we had wasn't so picky 
about such problems and the patch just tries to make the current code as 
robust as the old code was, which indeed seems to solve the problems we see.

The wrap around detection still works (tested by setting the initial 
fence value to 0xfffffff0 and letting it wrap around shortly after 
start), so I think it we can safely commit this.

Christian.