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Subject: Re: [PATCH v1] phy: qcom-qmp: Raise qcom_qmp_phy_enable() polling
 delay
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>
References: <92d97c68-d226-6290-37d6-f46f42ea604b@free.fr>
 <a3a50cf5-083a-5aa8-e77c-6feb2f2fd866@codeaurora.org>
 <134f4648-682e-5fed-60e7-bc25985dd7e9@free.fr>
From:   Kishon Vijay Abraham I <kishon@ti.com>
Message-ID: <e9d7667d-7ed4-d97e-b010-d61b214e6451@ti.com>
Date:   Thu, 20 Jun 2019 11:55:11 +0530
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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