linux-kernel - Re: [RFC PATCH v1 2/2] usb: host: add xhci-exynos to support Exynos SOCs

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Message-ID: <20221205022809.GC54922@ubuntu>
Date:   Mon, 5 Dec 2022 11:28:09 +0900
From:   Jung Daehwan <dh10.jung@...sung.com>
To:     Mathias Nyman <mathias.nyman@...ux.intel.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Alim Akhtar <alim.akhtar@...sung.com>,
        Mathias Nyman <mathias.nyman@...el.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Colin Ian King <colin.i.king@...il.com>,
        Artur Bujdoso <artur.bujdoso@...il.com>,
        Juergen Gross <jgross@...e.com>,
        Tomer Maimon <tmaimon77@...il.com>,
        "open list:USB SUBSYSTEM" <linux-usb@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        "moderated list:ARM/SAMSUNG S3C, S5P AND EXYNOS ARM ARCHITECTURES" 
        <linux-arm-kernel@...ts.infradead.org>,
        "open list:ARM/SAMSUNG S3C, S5P AND EXYNOS ARM ARCHITECTURES" 
        <linux-samsung-soc@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>, sc.suh@...sung.com,
        taehyun.cho@...sung.com, jh0801.jung@...sung.com,
        eomji.oh@...sung.com
Subject: Re: [RFC PATCH v1 2/2] usb: host: add xhci-exynos to support Exynos
 SOCs

On Fri, Dec 02, 2022 at 02:22:39PM +0200, Mathias Nyman wrote:
> On 1.12.2022 11.01, Arnd Bergmann wrote:
> >On Thu, Dec 1, 2022, at 09:06, Greg Kroah-Hartman wrote:
> >>On Thu, Dec 01, 2022 at 11:13:31AM +0900, Daehwan Jung wrote:
> >>>This driver works with xhci platform driver. It needs to override
> >>>functions of xhci_plat_hc_driver. Wakelocks are used for sleep/wakeup
> >>>scenario of system.
> >>
> >>So this means that no other platform xhci driver can be supported in the
> >>same system at the same time.
> >>
> >>Which kind of makes sense as that's not anything a normal system would
> >>have, BUT it feels very odd.  This whole idea of "override the platform
> >>driver" feels fragile, why not make these just real platform drivers and
> >>have the xhci platform code be a library that the other ones can use?
> >>That way you have more control overall, right?
> 
> Agree that overriding the generic platform driver xhci_hc_platform_driver
> from this exynos driver is odd.
> 
> But I don't understand how this works.
> Where are the hcds created and added when this xhci-exonys driver binds to
> the device? all this driver does in probe is the overriding?
> 
> Am I missing something here?
> 

This works mainly with xhci platform driver. But xhci-exynos needs to override
some funtions. xhci-exynos probes first with override own functons and
it works with xhci platform driver.

> >
> >Agreed, having another layer here (hcd -> xhci -> xhcd_platform ->
> >xhcd_exynos) would fit perfectly well into how other SoC specific
> >drivers are abstracted. This could potentially also help reduce
> >the amount of code duplication between other soc specific variants
> >(mtk, tegra, mvebu, ...) that are all platform drivers but don't
> >share code with xhci-plat.c.
> >
> >Alternatively, it seems that all of the xhci-exynos support could
> >just be part of the generic xhci-platform driver: as far as I can
> >tell, none of the added code is exynos specific at all, instead it
> >is a generic xhci that is using the wakeup_source framework.
> 
> Sounds reasonable as well, and if some exynos specific code is needed
> then just create a xhci_plat_priv struct for exynos and pass it in
> of_device_id data like other vendors that use the generic
> xhci-platform driver do.
> 

I considered using existing overrides like xhci_plat_priv but I couldn't
find a solution. My driver invokes probing xhci platform driver in
source code not device tree. Allocation of platform device is done
in dwc3_host_init(usb/dwc3/host.c). That's why I can't pass device data
to xhci platform driver.

> -Mathias
> 
>