From mboxrd@z Thu Jan 1 00:00:00 1970 From: Sinan Kaya Subject: Re: [PATCH v5 3/5] bnx2x: Eliminate duplicate barriers on weakly-ordered archs Date: Fri, 23 Mar 2018 12:51:47 -0400 Message-ID: <4bd9ccd2-df8f-acad-2513-eefe065dc852@codeaurora.org> References: <1521738603-23596-4-git-send-email-okaya@codeaurora.org> <20180323.122035.1380806748695640531.davem@davemloft.net> <20180323.124326.2170503491903886041.davem@davemloft.net> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20180323.124326.2170503491903886041.davem@davemloft.net> Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org To: David Miller Cc: netdev@vger.kernel.org, timur@codeaurora.org, sulrich@codeaurora.org, linux-arm-msm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, ariel.elior@cavium.com, everest-linux-l2@cavium.com, linux-kernel@vger.kernel.org List-Id: linux-arm-msm@vger.kernel.org On 3/23/2018 12:43 PM, David Miller wrote: > From: Sinan Kaya > Date: Fri, 23 Mar 2018 12:31:12 -0400 > >> Sorry, you got me confused now. >> >> If you look at the code closer, you'll see this. >> >> wmb(); >> >> txdata->tx_db.data.prod += nbd; >> barrier(); >> >> DOORBELL(bp, txdata->cid, txdata->tx_db.raw); >> >> and you also asked me to rename DOORBELL to DOORBELL_RELAXED() to make >> it obvious that we have a relaxed operator inside the macro. > > This still doesn't match the stated pattern. I can certainly update the commit text for this or spin into its own patch to make it obvious. > > wmb(); > /* no other memory or I/O or IOMEM operation */ > writel(); > > There is a write to a producer index there and then no non-compiler > barrier or any kind before the writel(). > > So, in fact, it might really need that implicit writel() barrier here! > It could if txdata->tx_db was not a union. There is a data dependency between txdata->tx_db.data.prod and txdata->tx_db.raw. So, no reordering. I can argue that barrier() here is useless in fact. Anyhow, I'll spin this piece out of this patch so that we pay special attention with a better description. -- 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.