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

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Date:   Fri, 26 Apr 2019 15:18:40 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Jon Hunter <jonathanh@...dia.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

26.04.2019 14:13, Jon Hunter пишет:
> 
> On 26/04/2019 11:45, Dmitry Osipenko wrote:
>> 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.
> 
> So why make the change?

For consistency.

>> 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?
> 
> If we can't quantify the performance gain, then it is difficult to
> justify the change. I would also be concerned if that is the only place
> we need an explicit dsb.

Maybe it won't hurt to add dsb to the ISR as well. But okay, let's drop
this patch for now.