Re: [v2 1/1] usb:host:xhci support option to disable xHCI 1.0 USB2 HW LPM

[Mathias Nyman <mathias.nyman@intel.com>]

On 05.06.2017 15:57, Thang Q. Nguyen wrote:
> On Mon, Jun 5, 2017 at 6:14 PM, Mathias Nyman
> <mathias.nyman@linux.intel.com> wrote:
>> On 20.05.2017 10:24, Thang Q. Nguyen wrote:
>>>
>>> XHCI specification 1.1 does not require xHCI 1.0 compliant controllers
>>> to always enable hardware USB2 LPM.
>>> However, the current xHCI driver always enable it by setting HLE=1 when
>>> seeing HLC=1. This makes certain xHCI controllers that have broken USB2
>>> HW LPM fail to work as there is no way to disable this feature.
>>> This patch adds support to control disabling USB2 Hardware LPM via
>>> DT/ACPI attribute.
>>>
>>
>>   Wouldn't it be better to just keep  xhci->hw_lpm_support = 0 if the host
>> doesn't support Hardware LPM Capability, (HLC)?
>>
>> This should prevent all those extra steps getting here just to do nothing.
> No, HLC = 0 means the host doesn't support Hardware LPM.
> The problem here is the host support Hardware LPM but there is a bug
> in host controller that make the LPM fail to work.
>

So the host support hw LPM, and has its HLC capability bit set,
but in the end it just doesn't work at all, and should be prevented.

> When debugging the host controller, we detect there are some holes in
> the current usb specifications which can can result in inter-operating
> problems between USB Host Controller and USB PHY. To be more specific,
> the specs have not clarified the resume recovery timing after the port
> has just waken up from L1. This can lead to different interpretations
> of the specs by Host Controller and PHY. What happened in our case is
> that a Host controller cannot work with a PHY right after resuming
> from L1 because these two Vendors have different views of the specs
> regarding LPM timing after L1. These views are contradictory and
> cannot work together.
>
> If Host Controller and PHY are from the same vendor, they might have
> some "internal handshake mechanisms" to avoid these holes of the USB
> specs. However, these mechanisms are not standardized in USB specs;
> and not all vendors follow these mechanisms. In fact, we have observed
> this kind of "internal handshake mechanism" in HOST Controller and PHY
> from SYNOPSYS DWC. So, we can say that if users use Host Controller
> and PHY from different Vendors, the inteopering problems after waking
> up from L1 are more likely to occur.

Can you explain the reason why you prefer preventing hw lpm inside the
xhci_set_usb2_hardware_lpm() function instead of preventing hw lpm usage
all together for this platform -i.e. by not setting xhci->hw_lpm_support


Again, something like:
if (temp & XHCI_HLC && !(xhci->quirks & XHCI_HW_LPM_BROKEN))
         xhci->hw_lpm_support = 1;

>> The HW LPM can also be disabled (per device) in sysfs if needed.
> This does not work. When the issue happens, the USB device is fail to
> probe so no /sys interface created. Messages displayed when issue
> happen is similar to:
>
> [ 2846.677903] usb 1-1: reset high-speed USB device number 2 using xhci-hcd
> [ 2882.037125] usb usb1-port1: Cannot enable. Maybe the USB cable is bad?
> or
> usb usb3-port1: disabled by hub (EMI?), re-enabling...
> [   57.840237] usb 3-1: USB disconnect, device number 5

Ok, the sysfs entry is not useful in this case.

-Mathias