Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756071AbbHYSZ0 (ORCPT ); Tue, 25 Aug 2015 14:25:26 -0400 Received: from mail-yk0-f176.google.com ([209.85.160.176]:36592 "EHLO mail-yk0-f176.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755520AbbHYSZW (ORCPT ); Tue, 25 Aug 2015 14:25:22 -0400 MIME-Version: 1.0 In-Reply-To: <1408ea96f7b8f33aef73e0f84abd8792.squirrel@www.codeaurora.org> References: <1440194989-28835-1-git-send-email-ygardi@codeaurora.org> <1440194989-28835-15-git-send-email-ygardi@codeaurora.org> <1408ea96f7b8f33aef73e0f84abd8792.squirrel@www.codeaurora.org> From: Rob Herring Date: Tue, 25 Aug 2015 13:25:02 -0500 Message-ID: Subject: Re: [PATCH v1 14/15] scsi: ufs: commit descriptors before setting the doorbell To: Yaniv Gardi Cc: linux-arm-msm , linux-scsi@vger.kernel.org, Dolev Raviv , Gilad Broner , linux-scsi-owner@vger.kernel.org, Jej B , Santosh Y , Subhash Jadavani , hch@infradead.org, Paul Bolle , "James E.J. Bottomley" , Vinayak Holikatti , "linux-kernel@vger.kernel.org" Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2265 Lines: 53 On Tue, Aug 25, 2015 at 7:36 AM, wrote: >> On Aug 21, 2015 3:10 PM, "Yaniv Gardi" wrote: >>> >>> Add a write memory barrier to make sure descriptors prepared are >>> actually >>> written to memory before ringing the doorbell. We have also added the >>> write memory barrier after ringing the doorbell register so that >>> controller sees the new request immediately. >>> >>> Signed-off-by: Yaniv Gardi >>> >>> --- >>> drivers/scsi/ufs/ufshcd.c | 6 ++++++ >>> 1 file changed, 6 insertions(+) >>> >>> diff --git a/drivers/scsi/ufs/ufshcd.c b/drivers/scsi/ufs/ufshcd.c >>> index fef0660..876148b 100644 >>> --- a/drivers/scsi/ufs/ufshcd.c >>> +++ b/drivers/scsi/ufs/ufshcd.c >>> @@ -833,6 +833,8 @@ void ufshcd_send_command(struct ufs_hba *hba, >>> unsigned int task_tag) >>> ufshcd_clk_scaling_start_busy(hba); >>> __set_bit(task_tag, &hba->outstanding_reqs); >>> ufshcd_writel(hba, 1 << task_tag, >>> REG_UTP_TRANSFER_REQ_DOOR_BELL); >>> + /* Make sure that doorbell is committed immediately */ >>> + wmb(); >> >> Is this really necessary? Is there a measurable difference? > > I'm not sure if there is a measurable difference, but as the Door-Bell > register is the one that actually responsible for the HW execution of the > requests, anyhow, it's recommended to its value will be written > instantly to the memory. A barrier doesn't guarantee speed, only ordering. Unless you can measure the difference, you should not have it. > Also, as the Interrupt context reads this register, and compare it to the > SW mirroring value (hba->outstanding_reqs) in order to realize what > requests are already completed, it's important to get the correct value > by reading this register, otherwise we might realize a request completion > while it was never even submitted. If a register read can pass a register write out of order, then your h/w is broken. Plus what if the interrupt occurs before the barrier. Rob -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/