linux-kernel - Re: [PATCH v1] dmaengine: tegra: Use relaxed versions of readl/writel

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Date:   Fri, 26 Apr 2019 10:52:15 +0100
From:   Jon Hunter <jonathanh@...dia.com>
To:     Dmitry Osipenko <digetx@...il.com>,
        Laxman Dewangan <ldewangan@...dia.com>,
        Vinod Koul <vkoul@...nel.org>,
        Thierry Reding <thierry.reding@...il.com>
CC:     <dmaengine@...r.kernel.org>, <linux-tegra@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1] dmaengine: tegra: Use relaxed versions of readl/writel

On 25/04/2019 00:17, Dmitry Osipenko wrote:
> The readl/writel functions are inserting memory barrier in order to
> ensure that memory stores are completed. On Tegra20 and Tegra30 this
> results in L2 cache syncing which isn't a cheapest operation. The
> tegra20-apb-dma driver doesn't need to synchronize generic memory
> accesses, hence use the relaxed versions of the functions.

Do you mean device-io accesses here as this is not generic memory?

Although there may not be any issues with this change, I think I need a
bit more convincing that we should do this given that we have had it
this way for sometime and I would not like to see us introduce any
regressions as this point without being 100% certain we would not.
Ideally, if I had some good extensive tests I could run to hammer the
DMA for all configurations with different combinations of channels
running simultaneously then we could test this, but right now I don't :-(

Have you ...
1. Tested both cyclic and scatter-gather transfers?
2. Stress tested simultaneous transfers with various different
   configurations?
3. Quantified the actual performance benefit of this change so we can
   understand how much of a performance boost this offers?

Cheers
Jon