From: Dmitry Osipenko <digetx@gmail.com>
To: Jon Hunter <jonathanh@nvidia.com>,
Laxman Dewangan <ldewangan@nvidia.com>,
Vinod Koul <vkoul@kernel.org>,
Thierry Reding <thierry.reding@gmail.com>
Cc: dmaengine@vger.kernel.org, linux-tegra@vger.kernel.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v1] dmaengine: tegra: Use relaxed versions of readl/writel
Date: Fri, 26 Apr 2019 13:45:29 +0300 [thread overview]
Message-ID: <4a315b63-bc71-3c3e-f1ae-8638bcf4033d@gmail.com> (raw)
In-Reply-To: <d789f1bc-44d1-abf6-a046-7cf835416e8f@nvidia.com>
26.04.2019 12:52, Jon Hunter пишет:
>
> 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?
Yes. The IOMEM accesses within are always ordered and uncached, while
generic memory accesses are out-of-order and cached.
> 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?
Actually I found a case where this change causes a problem, I'm seeing
I2C transfer timeout for touchscreen and it breaks the touch input.
Indeed, I haven't tested this patch very well.
And the fix is this:
@@ -1592,6 +1592,8 @@ static int tegra_dma_runtime_suspend(struct device
*dev)
TEGRA_APBDMA_CHAN_WCOUNT);
}
+ dsb();
+
clk_disable_unprepare(tdma->dma_clk);
return 0;
Apparently the problem is that CLK/DMA (PPSB/APB) accesses are
incoherent and CPU disables clock before writes are reaching DMA controller.
I'd say that cyclic and scatter-gather transfers are now tested. I also
made some more testing of simultaneous transfers.
Quantifying performance probably won't be easy to make as the DMA
read/writes are not on any kind of code's hot-path.
Jon, are you still insisting about to drop this patch or you will be
fine with the v2 that will have the dsb() in place?
next prev parent reply other threads:[~2019-04-26 10:45 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-24 23:17 [PATCH v1] dmaengine: tegra: Use relaxed versions of readl/writel Dmitry Osipenko
2019-04-26 9:52 ` Jon Hunter
2019-04-26 10:45 ` Dmitry Osipenko [this message]
2019-04-26 11:13 ` Jon Hunter
2019-04-26 12:18 ` Dmitry Osipenko
2019-04-26 12:42 ` Dmitry Osipenko
2019-04-26 13:03 ` Dmitry Osipenko
2019-04-26 15:11 ` Thierry Reding
2019-04-30 12:25 ` Dmitry Osipenko
2019-04-26 12:01 ` Vinod Koul
2019-04-26 12:19 ` Dmitry Osipenko
2019-04-26 12:47 ` Vinod Koul
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