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From: Kishon Vijay Abraham I <kishon@ti.com>
To: Marc Gonzalez <marc.w.gonzalez@free.fr>,
	Vivek Gautam <vivek.gautam@codeaurora.org>,
	Bjorn Andersson <bjorn.andersson@linaro.org>,
	Douglas Anderson <dianders@chromium.org>
Cc: MSM <linux-arm-msm@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v1] phy: qcom-qmp: Raise qcom_qmp_phy_enable() polling delay
Date: Thu, 20 Jun 2019 11:55:11 +0530
Message-ID: <e9d7667d-7ed4-d97e-b010-d61b214e6451@ti.com> (raw)
In-Reply-To: <134f4648-682e-5fed-60e7-bc25985dd7e9@free.fr>

Hi,

On 14/06/19 6:08 PM, Marc Gonzalez wrote:
> + Doug (who is familiar with usleep_range quirks)
> 
> On 14/06/2019 11:50, Vivek Gautam wrote:
> 
>> On 6/13/2019 5:02 PM, Marc Gonzalez wrote:
>>
>>> readl_poll_timeout() calls usleep_range() to sleep between reads.
>>> usleep_range() doesn't work efficiently for tiny values.
>>>
>>> Raise the polling delay in qcom_qmp_phy_enable() to bring it in line
>>> with the delay in qcom_qmp_phy_com_init().
>>>
>>> Signed-off-by: Marc Gonzalez <marc.w.gonzalez@free.fr>
>>> ---
>>> Vivek, do you remember why you didn't use the same delay value in
>>> qcom_qmp_phy_enable) and qcom_qmp_phy_com_init() ?
>>
>> phy_qcom_init() thingy came from the PCIE phy driver from downstream
>> msm-3.18 PCIE did something as below:
> 
> FWIW and IMO, drivers/pci/host/pci-msm.c is a good example of how not to write
> a device driver. It's huge (7000+ lines) because it handles multiple platforms
> via ifdefs, and lumps everything together (phy, core IP, SoC specific glue)
> in a single file.
> 
>> -----
>> do {
>>          if (pcie_phy_is_ready(dev))
>>                  break;
>>          retries++;
>>          usleep_range(REFCLK_STABILIZATION_DELAY_US_MIN,
>>                                   REFCLK_STABILIZATION_DELAY_US_MAX);
>> } while (retries < PHY_READY_TIMEOUT_COUNT);
>>
>> REFCLK_STABILIZATION_DELAY_US_MIN/MAX ==> 1000/1005
>> PHY_READY_TIMEOUT_COUNT ==> 10
>> -----
> 
> https://source.codeaurora.org/quic/la/kernel/msm-4.4/tree/drivers/pci/host/pci-msm.c?h=LE.UM.1.3.r3.25#n4624
> 
> https://source.codeaurora.org/quic/la/kernel/msm-4.4/tree/drivers/pci/host/pci-msm.c?h=LE.UM.1.3.r3.25#n1721
> 
> readl_relaxed(dev->phy + PCIE_N_PCS_STATUS(dev->rc_idx, dev->common_phy)) & BIT(6)
> is equivalent to:
> the check in qcom_qmp_phy_enable()
> 
> readl_relaxed(dev->phy + PCIE_COM_PCS_READY_STATUS) & 0x1
> is equivalent to:
> the check in qcom_qmp_phy_com_init()
> 
> I'll take a closer look, using some printks, to narrow down the run-time
> execution path.
> 
>> phy_enable() from the usb phy driver from downstream.
>>   /* Wait for PHY initialization to be done */
>>   do {
>>           if (readl_relaxed(phy->base +
>>                   phy->phy_reg[USB3_PHY_PCS_STATUS]) & PHYSTATUS)
>>                   usleep_range(1, 2);
>> else
>> break;
>>   } while (--init_timeout_usec);
>>
>> init_timeout_usec ==> 1000
>> -----
>> USB never had a COM_PHY status bit.
>>
>> So clearly the resolutions were different.
>>
>> Does this change solve an issue at hand?
> 
> The issue is usleep_range() being misused ^_^
> 
> Although usleep_range() takes unsigned longs as parameters, it is
> not appropriate over the entire 0-2^64 range.
> 
> a) It should not be used with tiny values, because the cost of programming
> the timer interrupt, and processing the resulting IRQ would dominate.
> 
> b) It should not be used with large values (above 2000000/HZ) because
> msleep() is more efficient, and is acceptable for these ranges.

Documentation/timers/timers-howto.txt has all the information on the various
kernel delay/sleep mechanisms. For < ~10us, it recommends to use udelay
(readx_poll_timeout_atomic). Depending on the actual timeout to be used, the
delay mechanism in timers-howto.txt should be used.

Thanks
Kishon

  reply index

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-06-13 11:32 Marc Gonzalez
2019-06-14  9:50 ` Vivek Gautam
2019-06-14 12:38   ` Marc Gonzalez
2019-06-20  6:25     ` Kishon Vijay Abraham I [this message]
2019-06-24 11:55       ` Marc Gonzalez
2019-06-24 15:52 ` Doug Anderson

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