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

[Mathias Nyman]

On 05.06.2017 15:57, Thang Q. Nguyen wrote:
> On Mon, Jun 5, 2017 at 6:14 PM, Mathias Nyman
> <mathias.nyman@xxxxxxxxxxxxxxx> 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