linux-kernel - Re: [PATCH v1 2/2] usb: tegra: Move UTMI-pads reset from ehci-tegra to tegra-phy

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Message-ID: <886b6cfe-1691-508b-335b-6f015747dd40@gmail.com>
Date:   Mon, 11 Dec 2017 16:31:39 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Thierry Reding <thierry.reding@...il.com>
Cc:     Felipe Balbi <balbi@...nel.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jonathan Hunter <jonathanh@...dia.com>,
        linux-usb@...r.kernel.org, linux-tegra@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 2/2] usb: tegra: Move UTMI-pads reset from ehci-tegra
 to tegra-phy

On 11.12.2017 13:25, Thierry Reding wrote:
> On Mon, Dec 11, 2017 at 02:07:38AM +0300, Dmitry Osipenko wrote:
>> UTMI pads are shared by USB controllers and reset of UTMI pads is shared
>> with the reset of USB1 controller. Currently reset of UTMI pads is done by
>> the EHCI driver and ChipIdea UDC works because EHCI driver always happen
>> to be probed first. Move reset controls from ehci-tegra to tegra-phy in
>> order to resolve the problem.
>>
>> Signed-off-by: Dmitry Osipenko <digetx@...il.com>
>> ---
>>  drivers/usb/host/ehci-tegra.c     | 87 ++++++++++++++++++---------------------
>>  drivers/usb/phy/phy-tegra-usb.c   | 46 +++++++++++++++++++++
>>  include/linux/usb/tegra_usb_phy.h |  2 +
>>  3 files changed, 87 insertions(+), 48 deletions(-)
> 
> I don't think we can do this. For one I don't think shared resets are
> going to work here because you really won't ever be able to reset after
> two devices have requested the same reset.

Ah, indeed. Originally I had the reset being done in the probe, but then changed
it in the last minute without proper testing. Good catch! I'll revert back patch
to the origin.

 Second, utmip_pad_close()
> could be called at any point and it will have the side-effect of either
> not doing a reset at all (because it is shared) or resetting the USBD
> controller at the same time.

utmip_pad_close() is only called on tegra-phy driver removal, so it is
absolutely fine.

> We've been over this code a great deal over the years. I'd love it to be
> simpler, but every time we tried to simplify it, things broke.

Well, the current code is already broken quite severely because now we have two
users of the tegra-phy: ehci-tegra and chipidea-tegra. Things brake if host
driver is loaded after the UDC because host would reset the UDC. And also pads
won't be reset if ehci-tegra isn't loaded at all.

Shared reset seems to be a perfect solution for us and of course it requires
extra carefulness.