linux-kernel - Re: [PATCH v4 5/6] IB/nes: Eliminate duplicate barriers on weakly-ordered archs

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Message-ID: <3b96019a-2b3b-374a-453e-0553fb04c0a7@codeaurora.org>
Date:   Tue, 20 Mar 2018 10:23:16 -0500
From:   Sinan Kaya <okaya@...eaurora.org>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     linux-rdma@...r.kernel.org, timur@...eaurora.org,
        sulrich@...eaurora.org, linux-arm-msm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Faisal Latif <faisal.latif@...el.com>,
        Doug Ledford <dledford@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 5/6] IB/nes: Eliminate duplicate barriers on
 weakly-ordered archs

On 3/20/2018 9:54 AM, Jason Gunthorpe wrote:
> On Mon, Mar 19, 2018 at 10:47:47PM -0400, Sinan Kaya wrote:
>> Code includes barrier() 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.
>>
>> Create a new wrapper function with relaxed write operator. Use the new
>> wrapper when a write is following a barrier().
>>
>> Signed-off-by: Sinan Kaya <okaya@...eaurora.org>
>>  drivers/infiniband/hw/nes/nes.h       |  5 +++++
>>  drivers/infiniband/hw/nes/nes_hw.c    | 21 ++++++++++++++-------
>>  drivers/infiniband/hw/nes/nes_mgt.c   | 15 ++++++++++-----
>>  drivers/infiniband/hw/nes/nes_nic.c   |  2 +-
>>  drivers/infiniband/hw/nes/nes_utils.c |  3 ++-
>>  drivers/infiniband/hw/nes/nes_verbs.c |  5 +++--
>>  6 files changed, 35 insertions(+), 16 deletions(-)
>>
>> diff --git a/drivers/infiniband/hw/nes/nes.h b/drivers/infiniband/hw/nes/nes.h
>> index 00c27291..85e007d 100644
>> +++ b/drivers/infiniband/hw/nes/nes.h
>> @@ -387,6 +387,11 @@ static inline void nes_write_indexed(struct nes_device *nesdev, u32 reg_index, u
>>  	spin_unlock_irqrestore(&nesdev->indexed_regs_lock, flags);
>>  }
>>  
>> +static inline void nes_write32_relaxed(void __iomem *addr, u32 val)
>> +{
>> +	writel_relaxed(val, addr);
>> +}
> 
> This wrapper is pointless, let us not add more..
> 
>>  static inline void nes_write32(void __iomem *addr, u32 val)
>>  {
>>  	writel(val, addr);
>> diff --git a/drivers/infiniband/hw/nes/nes_hw.c b/drivers/infiniband/hw/nes/nes_hw.c
>> index 18a7de1..568e17d 100644
>> +++ b/drivers/infiniband/hw/nes/nes_hw.c
>> @@ -1257,7 +1257,8 @@ int nes_destroy_cqp(struct nes_device *nesdev)
>>  
>>  	barrier();
>>  	/* Ring doorbell (5 WQEs) */
>> -	nes_write32(nesdev->regs+NES_WQE_ALLOC, 0x05800000 | nesdev->cqp.qp_id);
>> +	nes_write32_relaxed(nesdev->regs+NES_WQE_ALLOC,
>> +			    0x05800000 | nesdev->cqp.qp_id);
> 
> barrier() is not strong enough to order writel, so this doesn't seem
> right?
> 
> It is probably noteven strong enough for what this driver thinks it is
> doing..  This driver is essentially dead and broken, probably just
> don't change it.

Just for the sake of other changes in netdev directory and my education...

barrier() on ARM is a wmb(). It should be a compiler barrier on intel.

You are saying barrier() should have been a wmb(), right?

I have gone through similar exercise on netdev directory and changed

barrier()
writel()

to 

barrier()
writel_relaxed()

Do you see any problem with this?

If the goal is to make memory changes observable to the hardware, it should
have been, right not barrier()?

wmb()
writel_relaxed()

> 
> 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.