[U-Boot] [PATCH] usb: hub: don't check CONNECTION in hub_port_reset()

[Marek Vasut]

On Friday, August 08, 2014 at 01:07:59 AM, Stephen Warren wrote:
> From: Stephen Warren <swarren at nvidia.com>
> 
> One specific USB 3.0 device behaves strangely when reset by
> usb_new_device()'s call to hub_port_reset(). For some reason, the device
> appears to briefly drop off the bus when this second bus reset is
> executed, yet if we retry this loop, it'll eventually come back after
> another two resets.
> 
> If USB bus reset is executed over and over within usb_new_device()'s call
> to hub_port_reset(), I see the following sequence of results, which
> repeats as long as you want:
> 
> 1) STAT_C_CONNECTION = 1 STAT_CONNECTION = 0  USB_PORT_STAT_ENABLE 0
> 2) STAT_C_CONNECTION = 1 STAT_CONNECTION = 1  USB_PORT_STAT_ENABLE 0
> 3) STAT_C_CONNECTION = 1 STAT_CONNECTION = 1  USB_PORT_STAT_ENABLE 1
> 
> The device in question is a SanDisk Ultra USB 3.0 16GB memory stick with
> USB VID/PID 0x0781/0x5581.
> 
> In order to allow this device to work with U-Boot, ignore the
> {C_,}CONNECTION bits in the status/change registers, and only use the
> ENABLE bit to determine if the reset was successful.
> 
> To be honest, extensive investigation has failed to determine why this
> problem occurs. I'd love to know! I don't know if it's caused by:
> * A HW bug in the device
> * A HW bug in the Tegra USB controller
> * A SW bug in the U-Boot Tegra USB driver
> * A SW bug in the U-Boot USB core
> 
> This issue only occurs when the device's USB3 pins are attached to the
> host; if only the USB2 pins are connected the issue does not occur. The
> USB3 controller on Tegra is in reset, so is not actively communicating
> with the device at all - a USB3 analyzer confirms this. Slightly
> unplugging the device (so the USB3 pins don't contact) or using a USB2
> cable or hub as an intermediary avoids the problem. For some reason,
> the Linux kernel (either on the same Tegra board, or on an x86 host)
> has no issue with the device, and I observe no disconnections during
> reset.
> 
> This change won't affect any USB device that already works, since such
> devices could not currently be triggering the error return this patch
> removes, or they wouldn't be working currently.
> 
> However, this patch is quite reliable in practice, hence I hope it's
> acceptable to solve the problem.
> 
> The only potential fallout I can see from this patch is:
> 
> * A broken device that triggers C_CONNECTION/!CONNECTION now causes the
>   loop in hub_port_reset() to run multiple times. If it never succeeds,
>   this will cause "usb start" to take roughly 1s extra to execute.
> 
> * If the user unplugs a device while hub_port_reset() is executing, and
>   very quickly swaps in a new device, hub_port_reset() might succeed on
>   the new device. This would mean that any information cached about the
>   original device (from the descriptor read in usb_new_device(), which
>   simply caches the max packet size) might be invalid, which would cause
>   problems talking to the new device. However, without this change, the
>   new device wouldn't work anyway, so this is probably not much of a
>   loss.
> 
> Signed-off-by: Stephen Warren <swarren at nvidia.com>
> ---

I'll apply this and see if we get any breakage. I don't expect any though.