linux-kernel - Re: [PATCH 2/6] dma: bcm-sba-raid: Peek mbox when we are left with no free requests

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Date:   Fri, 28 Jul 2017 08:45:01 +0530
From:   Vinod Koul <vinod.koul@...el.com>
To:     Anup Patel <anup.patel@...adcom.com>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Dan Williams <dan.j.williams@...el.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Scott Branden <sbranden@...adcom.com>,
        Ray Jui <rjui@...adcom.com>,
        Linux Kernel <linux-kernel@...r.kernel.org>,
        Linux ARM Kernel <linux-arm-kernel@...ts.infradead.org>,
        Device Tree <devicetree@...r.kernel.org>,
        dmaengine@...r.kernel.org,
        BCM Kernel Feedback <bcm-kernel-feedback-list@...adcom.com>
Subject: Re: [PATCH 2/6] dma: bcm-sba-raid: Peek mbox when we are left with
 no free requests

On Thu, Jul 27, 2017 at 10:25:25AM +0530, Anup Patel wrote:
> On Wed, Jul 26, 2017 at 10:40 PM, Vinod Koul <vinod.koul@...el.com> wrote:
> > On Wed, Jul 26, 2017 at 11:06:40AM +0530, Anup Patel wrote:
> >> We should peek mbox channels when we are left with no free
> >> sba_requests in sba_alloc_request()
> >
> > and why is the world should we do that, how does that help??
> 
> When setting up RAID array on several NVMe disk we observed
> that sba_alloc_request() start failing (due to no free requests left)
> and RAID array setup becomes very slow.
> 
> Doing mbox channel peek when we have no free requests left,
> improves performance of RAID array setup.

How about documenting this tribal knowledge in the changelog. Changelogs are
very useful, 6 months down the line, you will struggle to remember why this
was changed..

> 
> This change is inspired from mv_chan_alloc_slot() implemented
> in drivers/dma/mv_xor.c
> 
> Regards,
> Anup

-- 
~Vinod