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Subject: Re: [PATCH v4 0/2] ARM: sun8i-r40: Enable usb otg support
From:   Icenowy Zheng <icenowy@aosc.io>
To:     Evgeny Boger <boger@wirenboard.com>, qianfanguijin@163.com
Cc:     andre.przywara@arm.com, devicetree@vger.kernel.org,
        jernej.skrabec@gmail.com, krzysztof.kozlowski+dt@linaro.org,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-sunxi@lists.linux.dev, robh+dt@kernel.org,
        samuel@sholland.org, wens@csie.org
Date:   Mon, 23 May 2022 21:10:09 +0800
In-Reply-To: <ec5f7fe8-d47d-ce7f-0e0a-2bdf41a88ba2@wirenboard.com>
References: <264cb004-677a-13df-cc68-676ef3c2e7d8@163.com>
         <ec5f7fe8-d47d-ce7f-0e0a-2bdf41a88ba2@wirenboard.com>
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在 2022-05-19星期四的 23:54 +0300，Evgeny Boger写道：
> Hi qianfan,
> 
> As Allwinner A40i user, let me first thank you for your effort for 
> making better upstream support for R40!
> 
> However, I would strongly suggest *not* to add USB support to one
> more 
> Allwinner SoC in this particular way.
> The problem is, this approach consists of a number of carefully
> crafted 
> hacks in device tree to make current drivers work on Allwinner
> hardware 
> without modification to the drivers.
> 
> a few examples:
> 
> 1) please notice how ohci0 and ehci0 nodes do not contain reference
> to 
> usb phy. It is done intentionally, otherwise EHCI will reset musb
> mode.
> Of course omitting phy reference here is also completely breaking
> power 
> cycling in case of usb error and otherwise messes with a power
> management.
> 
> 2) one must always enable ohci, ehci and usb_otg nodes at the same
> time. 
> If one forgets to enable ohci/ehci nodes while enabling usb_otg node,
> the system will silently fail to work as USB host.
> 
> 3) For host-only mode we still have to enable usb_otg node despite no
> role switching is needed. That's because phy reference is missing in 
> ehci/ohci, so the ehci/ohci driver won't enable the PHY.
> Also I might be wrong, but I think phy won't be routed to ehci/ohci 
> controllers is this case.
> 
> 4) musb host controller is initialized and present to hardware though
> never actually used
> 
> To summarize, not only the resulting device tree is not describing
> the 
> hardware properly, it is creating device tree configuration which
> will 
> be very hard to support in future, once proper driver support is in
> place.
> 
> 
> At Wiren Board kernel tree we tried to untangle this issue [1-6]. 
> Unfortunately I didn't have time to prepare it for kernel submission 
> yet, but I think I better submit it as RFC to get a feedback from you
> and others.

Please cc me in the following patches because it's me who makes the
whole dual route thing hacky.

> 
> 
> [1] 
> https://github.com/wirenboard/linux/commit/359abbbd86ddff4d3c61179c882c286de32bb089
> [2] 
> https://github.com/wirenboard/linux/commit/6327f9d7972c21b229fb83457fdde643b31553f9
> [3] 
> https://github.com/wirenboard/linux/commit/f01f4c66758bde460a4d8c5b54ecee3b585c0232
> [4] 
> https://github.com/wirenboard/linux/commit/c27598ad601e5a46f624b73412a531d6f1f63d37
> [5] 
> https://github.com/wirenboard/linux/commit/5796d6eebb86b32a3751b2038b63af46f94954b3
> [6] 
> https://github.com/wirenboard/linux/commit/0928a675d875f9c2849fd3a9888f718bbb673bda
> 
>