Message-ID: <20200714093239.4kvonvkwkdwobeia@macbook-pro.local>
Date:   Tue, 14 Jul 2020 11:32:39 +0200
From:   Willy Wolff <willy.mh.wolff.ml@...il.com>
To:     Lukasz Luba <lukasz.luba@....com>
Cc:     Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-samsung-soc@...r.kernel.org, k.konieczny@...sung.com,
        cw00.choi@...sung.com, krzk@...nel.org, chanwoo@...nel.org,
        myungjoo.ham@...sung.com, kyungmin.park@...sung.com,
        s.nawrocki@...sung.com, kgene@...nel.org
Subject: Re: [PATCH v2 2/2] memory: samsung: exynos5422-dmc: Add module param
 to control IRQ mode

Hi Lukasz and Bartek,

On 2020-07-14-10-01-16, Lukasz Luba wrote:
>Hi Bartek,
>
>On 7/14/20 8:42 AM, Bartlomiej Zolnierkiewicz wrote:
>>
>>Hi,
>>
>>On 7/10/20 9:11 PM, Lukasz Luba wrote:
>>>The driver can operate in two modes relaying on devfreq monitoring
>>>mechanism which periodically checks the device status or it can use
>>>interrupts when they are provided by loaded Device Tree. The newly
>>>introduced module parameter can be used to choose between devfreq
>>>monitoring and internal interrupts without modifying the Device Tree.
>>>It also sets devfreq monitoring as default when the parameter is not set
>>>(also the case for default when the driver is not built as a module).
>>
>>Could you please explain why should we leave the IRQ mode
>>support in the dmc driver?
>
>I am still experimenting with the IRQ mode in DMC, but have limited time
>for it and no TRM.
>The IRQ mode in memory controller or bus controller has one major
>advantage: is more interactive. In polling we have fixed period, i.e.
>100ms - that's a lot when we have a sudden, latency sensitive workload.
>There might be no check of the device load for i.e. 99ms, but the tasks
>with such workload started running. That's a long period of a few frames
>which are likely to be junked. Should we adjust polling interval to i.e.
>10ms, I don't think so. There is no easy way to address all of the
>scenarios.
>
>>
>>What are the advantages over the polling mode?
>
>As described above: more reactive to sudden workload, which might be
>latency sensitive and cause junk frames.
>Drawback: not best in benchmarks which are randomly jumping
>over the data set, causing low traffic on memory.
>It could be mitigated as Sylwester described with not only one type
>of interrupt, but another, which could 'observe' also other information
>type in the counters and fire.
>
>>
>>In what scenarios it should be used?
>
>System like Android with GUI, when there is this sudden workload
>quite often.
>
>I think the interconnect could help here and would adjust the DMC
>freq upfront. Although I don't know if interconnect on Exynos5422 is in
>your scope in near future. Of course the interconnect will not cover
>all scenarios either.
>

The interconnect (CCI-400) will not help much, you still have the same problem
of setting interrupts at the right threshold, or to poll it to see any activity
through it.

>
>>
>>[ If this is only for documentation purposes then it should be
>>   removed as it would stay in (easily accessible) git history
>>   anyway.. ]
>
>The current interrupt mode is definitely not perfect and switching
>to devfreq monitoring mode has more sense. On the other hand, it
>still has potential, until there is no interconnect for this SoC.
>I will continue experimenting with irq mode, so I would like to
>still have the code in the driver.
>
>Regards,
>Lukasz
>
>>
>>Best regards,
>>--
>>Bartlomiej Zolnierkiewicz
>>Samsung R&D Institute Poland
>>Samsung Electronics
>>
>>>Reported-by: Willy Wolff <willy.mh.wolff.ml@...il.com>
>>>Signed-off-by: Lukasz Luba <lukasz.luba@....com>
>>>---
>>>  drivers/memory/samsung/exynos5422-dmc.c | 9 +++++++--
>>>  1 file changed, 7 insertions(+), 2 deletions(-)
>>>
>>>diff --git a/drivers/memory/samsung/exynos5422-dmc.c b/drivers/memory/samsung/exynos5422-dmc.c
>>>index e03ee35f0ab5..53bfe6b7b703 100644
>>>--- a/drivers/memory/samsung/exynos5422-dmc.c
>>>+++ b/drivers/memory/samsung/exynos5422-dmc.c
>>>@@ -12,6 +12,7 @@
>>>  #include <linux/io.h>
>>>  #include <linux/mfd/syscon.h>
>>>  #include <linux/module.h>
>>>+#include <linux/moduleparam.h>
>>>  #include <linux/of_device.h>
>>>  #include <linux/pm_opp.h>
>>>  #include <linux/platform_device.h>
>>>@@ -21,6 +22,10 @@
>>>  #include "../jedec_ddr.h"
>>>  #include "../of_memory.h"
>>>+static int irqmode;
>>>+module_param(irqmode, int, 0644);
>>>+MODULE_PARM_DESC(irqmode, "Enable IRQ mode (0=off [default], 1=on)");
>>>+
>>>  #define EXYNOS5_DREXI_TIMINGAREF		(0x0030)
>>>  #define EXYNOS5_DREXI_TIMINGROW0		(0x0034)
>>>  #define EXYNOS5_DREXI_TIMINGDATA0		(0x0038)
>>>@@ -1428,7 +1433,7 @@ static int exynos5_dmc_probe(struct platform_device *pdev)
>>>  	/* There is two modes in which the driver works: polling or IRQ */
>>>  	irq[0] = platform_get_irq_byname(pdev, "drex_0");
>>>  	irq[1] = platform_get_irq_byname(pdev, "drex_1");
>>>-	if (irq[0] > 0 && irq[1] > 0) {
>>>+	if (irq[0] > 0 && irq[1] > 0 && irqmode) {
>>>  		ret = devm_request_threaded_irq(dev, irq[0], NULL,
>>>  						dmc_irq_thread, IRQF_ONESHOT,
>>>  						dev_name(dev), dmc);
>>>@@ -1485,7 +1490,7 @@ static int exynos5_dmc_probe(struct platform_device *pdev)
>>>  	if (dmc->in_irq_mode)
>>>  		exynos5_dmc_start_perf_events(dmc, PERF_COUNTER_START_VALUE);
>>>-	dev_info(dev, "DMC initialized\n");
>>>+	dev_info(dev, "DMC initialized, in irq mode: %d\n", dmc->in_irq_mode);
>>>  	return 0;
>>>
>>

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