On Thu, May 26, 2016 at 05:23:30PM +0200, Thierry Reding wrote:
> From: Thierry Reding <treding@nvidia.com>
> 
> Starting with commit 0b52297f2288 ("reset: Add support for shared reset
> controls") there is a reference count for reset control assertions. The
> goal is to allow resets to be shared by multiple devices and an assert
> will take effect only when all instances have asserted the reset.
> 
> In order to preserve backwards-compatibility, all reset controls become
> exclusive by default. This is to ensure that reset_control_assert() can
> immediately assert in hardware.
> 
> However, this new behaviour triggers the following warning in the EHCI
> driver for Tegra:
> 
> [    3.365019] ------------[ cut here ]------------
> [    3.369639] WARNING: CPU: 0 PID: 1 at drivers/reset/core.c:187 __of_reset_control_get+0x16c/0x23c
> [    3.382151] Modules linked in:
> [    3.385214] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.6.0-rc6-next-20160503 #140
> [    3.392769] Hardware name: NVIDIA Tegra SoC (Flattened Device Tree)
> [    3.399046] [<c010fa50>] (unwind_backtrace) from [<c010b120>] (show_stack+0x10/0x14)
> [    3.406787] [<c010b120>] (show_stack) from [<c0347dcc>] (dump_stack+0x90/0xa4)
> [    3.414007] [<c0347dcc>] (dump_stack) from [<c011f4fc>] (__warn+0xe8/0x100)
> [    3.420964] [<c011f4fc>] (__warn) from [<c011f5c4>] (warn_slowpath_null+0x20/0x28)
> [    3.428525] [<c011f5c4>] (warn_slowpath_null) from [<c03cc8cc>] (__of_reset_control_get+0x16c/0x23c)
> [    3.437648] [<c03cc8cc>] (__of_reset_control_get) from [<c0526858>] (tegra_ehci_probe+0x394/0x518)
> [    3.446600] [<c0526858>] (tegra_ehci_probe) from [<c04516d8>] (platform_drv_probe+0x4c/0xb0)
> [    3.455029] [<c04516d8>] (platform_drv_probe) from [<c044fe78>] (driver_probe_device+0x1ec/0x330)
> [    3.463892] [<c044fe78>] (driver_probe_device) from [<c0450074>] (__driver_attach+0xb8/0xbc)
> [    3.472320] [<c0450074>] (__driver_attach) from [<c044e1ec>] (bus_for_each_dev+0x68/0x9c)
> [    3.480489] [<c044e1ec>] (bus_for_each_dev) from [<c044f338>] (bus_add_driver+0x1a0/0x218)
> [    3.488743] [<c044f338>] (bus_add_driver) from [<c0450768>] (driver_register+0x78/0xf8)
> [    3.496738] [<c0450768>] (driver_register) from [<c010178c>] (do_one_initcall+0x40/0x170)
> [    3.504909] [<c010178c>] (do_one_initcall) from [<c0c00ddc>] (kernel_init_freeable+0x158/0x1f8)
> [    3.513600] [<c0c00ddc>] (kernel_init_freeable) from [<c0810784>] (kernel_init+0x8/0x114)
> [    3.521770] [<c0810784>] (kernel_init) from [<c0107778>] (ret_from_fork+0x14/0x3c)
> [    3.529361] ---[ end trace 4bda87dbe4ecef8a ]---
> 
> The reason is that Tegra SoCs have three EHCI controllers, each with a
> separate reset line. However the first controller contains UTMI pads
> configuration registers that are shared with its siblings and that are
> reset as part of the first controller's reset. There is special code in
> the driver to assert and deassert this shared reset at probe time, and
> it does so irrespective of which controller is probed first to ensure
> that these shared registers are reset before any of the controllers are
> initialized. Unfortunately this means that if the first controller gets
> probed first, it will request its own reset line and will subsequently
> request the same reset line again (temporarily) to perform the reset.
> This used to work fine before the above-mentioned commit, but now
> triggers the new WARN.
> 
> Work around this by making sure we reuse the controller's reset if the
> controller happens to be the first controller.
> 
> Cc: Philipp Zabel <p.zabel@pengutronix.de>
> Cc: Hans de Goede <hdegoede@redhat.com>
> Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
> Signed-off-by: Thierry Reding <treding@nvidia.com>
> ---
> Changes in v4:
> - avoid calling reset_control_put() on ERR_PTR()-encoded error codes
> 
> Changes in v3:
> - reword commit message to more accurately describe the hardware design
> 
> Changes in v2:
> - restore has_utmi_pad_registers condition (Alan Stern)
> 
>  drivers/usb/host/ehci-tegra.c | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)

Alan, Greg,

I forgot to mention, but it'd be great if this could go into v4.7
because the reset framework commit that triggers this was merged into
Linus' tree last week.

Thierry