[PATCH v4 1/6] RDMA/bnxt_re: Eliminate duplicate barriers on weakly-ordered archs

[Sinan Kaya]

On 3/20/2018 9:48 AM, Jason Gunthorpe wrote:
> On Mon, Mar 19, 2018 at 10:47:43PM -0400, Sinan Kaya wrote:
>> Code includes wmb() followed by writel(). writel() already has a barrier on
>> some architectures like arm64.
>>
>> This ends up CPU observing two barriers back to back before executing the
>> register write.
>>
>> Since code already has an explicit barrier call, changing writel() to
>> writel_relaxed().
>>
>> Signed-off-by: Sinan Kaya <okaya at codeaurora.org>
>>  drivers/infiniband/hw/bnxt_re/qplib_rcfw.c | 8 ++++----
>>  1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/infiniband/hw/bnxt_re/qplib_rcfw.c b/drivers/infiniband/hw/bnxt_re/qplib_rcfw.c
>> index 8329ec6..4a6b981 100644
>> +++ b/drivers/infiniband/hw/bnxt_re/qplib_rcfw.c
>> @@ -181,10 +181,10 @@ static int __send_message(struct bnxt_qplib_rcfw *rcfw, struct cmdq_base *req,
>>  
>>  	/* ring CMDQ DB */
>>  	wmb();
>> -	writel(cmdq_prod, rcfw->cmdq_bar_reg_iomem +
>> -	       rcfw->cmdq_bar_reg_prod_off);
>> -	writel(RCFW_CMDQ_TRIG_VAL, rcfw->cmdq_bar_reg_iomem +
>> -	       rcfw->cmdq_bar_reg_trig_off);
>> +	writel_relaxed(cmdq_prod, rcfw->cmdq_bar_reg_iomem +
>> +		       rcfw->cmdq_bar_reg_prod_off);
>> +	writel_relaxed(RCFW_CMDQ_TRIG_VAL, rcfw->cmdq_bar_reg_iomem +
>> +		       rcfw->cmdq_bar_reg_trig_off);
> 
> Woah, this may not be safe..
> 
> The definition of writel_relaxed() is that it is fully unordered, so
> the above two writes may change order now. Broadcom guys would have to
> ack if that it is OK or not for their hardware.
> 
> In general this is not an OK approach for a mechanical
> conversion.. Only the first writel can be convereted.
> 
> You need to check all your patches to make sure there are no
> subsequent writel's in the places touched.

I paid special attention to this one and went to check the barriers
document. According to the document, writes (whether it is relaxed or not)
are always observed by the HW inorder with respect to each other.
It just doesn't guarantee anything above writel_relaxed() to be observed.
Since we already have a wmb(), this is taken care of. 

If somebody wants things to be observed after register write, there should
have been a wmb() or mmiowb() afterwards.


> 
> Jason
> 


-- 
Sinan Kaya
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.