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@xxxxxxx>
>>> ---
>>> 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
