From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id CA223CEF174 for ; Tue, 8 Oct 2024 13:47:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=+OB9wPSXtzymu+56697yfFjidosVyjvoz1Sxeb8Kw4I=; b=EScF6dg0yoBp2ZkEfLDyZK/2DO BPOCQHO0TpkTqnvp0gSbB4o9TwFbnzONWNWsolLSf4vJ5u3bOyNfK011+3VHqDYu6BTEJDTrSArcw xF++EWM5WvhGSUGPm5sMDnXCAV3YXujyCdVN7afCsLyUtY4QerlefNwQThb/ZqoHHv/aW5uHrhKkx eqGhL7IVy/9XxPZIAO/pjzEr/oTh0Fk+ir5Gdh+CsgDII5Y5hyS+CSmn6Gu1f3hIk/KZE9BcqjQBr at/V0GfggdSJTtnu+Q4ZMmGw2TQmFD5A9FwCmITXs32JuYYS2oyLkcAeFZStPP0pfPyaNssf6ASCF OxiCrjbg==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98 #2 (Red Hat Linux)) id 1syAYb-000000063SE-2jBl; Tue, 08 Oct 2024 13:47:21 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.98 #2 (Red Hat Linux)) id 1syAMi-000000060qV-3Uaw for linux-arm-kernel@lists.infradead.org; Tue, 08 Oct 2024 13:35:06 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 01F3ADA7; Tue, 8 Oct 2024 06:35:33 -0700 (PDT) Received: from bogus (e133711.arm.com [10.1.196.77]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 5A0273F58B; Tue, 8 Oct 2024 06:35:02 -0700 (PDT) Date: Tue, 8 Oct 2024 14:34:59 +0100 From: Sudeep Holla To: Cristian Marussi Cc: Justin Chen , arm-scmi@vger.kernel.org, linux-arm-kernel@lists.infradead.org, peng.fan@nxp.com, Sudeep Holla , bcm-kernel-feedback-list@broadcom.com, florian.fainelli@broadcom.com Subject: Re: [PATCH] firmware: arm_scmi: Queue in scmi layer for mailbox implementation Message-ID: References: <20241004221257.2888603-1-justin.chen@broadcom.com> <1ad5c4e9-9f98-40ab-afa4-a7939781e8cc@broadcom.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20241008_063504_997457_AD49BEF2 X-CRM114-Status: GOOD ( 34.75 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Tue, Oct 08, 2024 at 02:23:00PM +0100, Sudeep Holla wrote: > On Tue, Oct 08, 2024 at 01:10:39PM +0100, Cristian Marussi wrote: > > On Mon, Oct 07, 2024 at 10:58:47AM -0700, Justin Chen wrote: > > > Thanks for the response. I'll try to elaborate. > > > > > > When comparing SMC and mailbox transport, we noticed mailbox transport > > > timesout much quicker when under load. Originally we thought this was the > > > latency of the mailbox implementation, but after debugging we noticed a > > > weird behavior. We saw SMCI transactions timing out before the mailbox even > > > transmitted the message. > > > > > > This issue lies in the SCMI layer. drivers/firmware/arm_scmi/driver.c > > > do_xfer() function. > > > > > > The fundamental issue is send_message() blocks for SMC transport, but > > > doesn't block for mailbox transport. So if send_message() doesn't block we > > > can have multiple messages waiting at scmi_wait_for_message_response(). > > > > > > > oh...yes...now I can see it...tx_prepare is really never called given > > how the mailbox subsystem de-queues messages once at time...so we end up > > waiting for a reply to some message that is still to be sent...so the > > message inflight is really NOT corrupted because the next remain pending > > until the reply in the shmem is read back , BUT the timeout will drift away > > if you multiple inflights are pending to be sent... > > > > Indeed. > > > > SMC looks like this > > > CPU #0 SCMI message 0 -> calls send_message() then calls > > > scmi_wait_for_message_response(), timesout after 30ms. > > > CPU #1 SCMI message 1 -> blocks at send_message() waiting for SCMI message 0 > > > to complete. > > > > > > Mailbox looks like this > > > CPU #0 SCMI message 0 -> calls send_message(), mailbox layer queues up > > > message, mailbox layer sees no message is outgoing and sends it. CPU waits > > > at scmi_wait_for_message_response(), timesout after 30ms > > > CPU #1 SCMI message 1 -> calls send_message(), mailbox layer queues up > > > message, mailbox layer sees message pending, hold message in queue. CPU > > > waits at scmi_wait_for_message_response(), timesout after 30ms. > > > > > > Lets say if transport takes 25ms. The first message would succeed, the > > > second message would timeout after 5ms. > > > > > > Hopefully this makes sense. > > > > Yes, of course, thanks, for reporting this, and taking time to > > explain... > > > > ...in general the patch LGTM...I think your patch is good also because it > > could be easily backported as a fix....can you add a Fixes tag in your > > next version ? > > > > Are you seeing this issue a lot ? IOW, do we need this to be backported ? > > > Also can you explain in more detail the issue and the solution in the commit > > message....that will help having it merged as a Fix in stables... > > > > ...for the future (definitely NOT in this series) we could probably think to > > get rid of the sleeping mutex in favour of some other non-sleeping form of > > mutual exclusion around the channnel (like in SMC transport) and enable > > (optionally) Atomic transmission support AND also review if the shmem > > layer busy-waiting in txprepare is anymore needed at all... > > > > Agreed, if we are locking the channel in SCMI, we can drop the busy-waiting > in tx_prepare and the associated details in the comment as this locking > voids that. It is better have both the changes in the same patch to indicate > the relation between them. Actually scratch that last point. The waiting in tx_prepare until the platform marks it free for agent to use is still needed. One usecase is when agent/OS times out but platform continues to process and eventually releases the shmem. Sorry I completely forgot about that. -- Regards, Sudeep