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

[Thierry Reding]

On Fri, Apr 26, 2019 at 04:03:08PM +0300, Dmitry Osipenko wrote:
> 26.04.2019 15:42, Dmitry Osipenko ÐÐÑÐÑ:
> > 26.04.2019 15:18, Dmitry Osipenko ÐÐÑÐÑ:
> >> 26.04.2019 14:13, Jon Hunter ÐÐÑÐÑ:
> >>>
> >>> On 26/04/2019 11:45, Dmitry Osipenko wrote:
> >>>> 26.04.2019 12:52, Jon Hunter ÐÐÑÐÑ:
> >>>>>
> >>>>> 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.
> >>
> > 
> > Jon, it occurred to me that there still should be a problem with the
> > writel() ordering in the driver because writel() ensures that memory
> > stores are completed *before* the write occurs and hence translates into
> > iowmb() + writel_relaxed() [0]. Thus the last write will always happen
> > asynchronously in regards to clk accesses.
> > 
> > [0]
> > https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tree/arch/arm/include/asm/io.h#n311
> > 
> 
> Also please note that iowmb() translates into wmb() if
> CONFIG_ARM_DMA_MEM_BUFFERABLE=y and sometime ago I was profiling host1x
> driver job submission performance and have seen cases where wmb() could
> take up to 1ms on T20 due to L2 syncing if there are outstanding memory
> writes in the cache (or even more, I don't remember exactly already how
> bad it was..).

This looks to be primarily caused by the fact that we have the L2X0
cache on Tegra20. So there's not really anything that can be done there
without potentially compromising correctness of the code.

> Altogether, I think the usage of readl/writel in pretty much all of
> Tegra drivers is plainly wrong and explicit dsb() shall be used in
> places where hardware synchronization is really needed.

I don't think that's an accurate observation. readl()/writel() are more
likely to be correct than the relaxed versions. You already saw yourself
that using the relaxed versions can easily introduce regressions.

Granted, readl()/writel() might add more memory barriers than strictly
necessary, and therefore they might in many cases be suboptimal. But, we
can't just go and engage in a wholesale conversion of all drivers. If we
do this, we need to very carefully audit every conversion to make sure
no regressions are introduced. This is especially complicated because
these would be subtle regressions and may be difficult to catch or
reproduce.

Also, we should avoid using primitives such as dsb in driver code to
avoid making the code too architecture specific.

Thierry

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