Message-ID: <0a9c19bd-0d25-1035-57e3-b1f5f204c309@redhat.com>
Date:   Tue, 8 Dec 2020 10:36:00 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     "Enrico Weigelt, metux IT consult" <lkml@...ux.net>,
        "Enrico Weigelt, metux IT consult" <info@...ux.net>,
        linux-kernel@...r.kernel.org, corbet@....net,
        linus.walleij@...aro.org, bgolaszewski@...libre.com,
        linux-doc@...r.kernel.org, linux-gpio@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-riscv@...ts.infradead.org, stefanha@...hat.com,
        msuchanek@...e.de
Subject: Re: [PATCH v2 2/2] drivers: gpio: add virtio-gpio guest driver


On 2020/12/7 下午9:53, Michael S. Tsirkin wrote:
> On Mon, Dec 07, 2020 at 11:12:50AM +0800, Jason Wang wrote:
>> On 2020/12/6 上午3:32, Michael S. Tsirkin wrote:
>>> On Sat, Dec 05, 2020 at 08:59:55AM +0100, Enrico Weigelt, metux IT consult wrote:
>>>> On 04.12.20 04:35, Jason Wang wrote:
>>>>
>>>>>> --- a/drivers/gpio/Kconfig
>>>>>> +++ b/drivers/gpio/Kconfig
>>>>>> @@ -1615,6 +1615,15 @@ config GPIO_MOCKUP
>>>>>>         tools/testing/selftests/gpio/gpio-mockup.sh. Reference the
>>>>>> usage in
>>>>>>         it.
>>>>>>   +config GPIO_VIRTIO
>>>>>> +    tristate "VirtIO GPIO support"
>>>>>> +    depends on VIRTIO
>>>>> Let's use select, since there's no prompt for VIRTIO and it doesn't have
>>>>> any dependencies.
>>>> whoops, it's not that simple:
>>>>
>>>> make: Entering directory '/home/nekrad/src/apu2-dev/pkg/kernel.apu2.git'
>>>> make[1]: Entering directory
>>>> '/home/nekrad/src/dk/DistroKit/platform-x86_64/build-target/linux-5.8.9-build'
>>>>     GEN     Makefile
>>>> drivers/gpu/drm/Kconfig:74:error: recursive dependency detected!
>>>> drivers/gpu/drm/Kconfig:74:	symbol DRM_KMS_HELPER is selected by
>>>> DRM_VIRTIO_GPU
>>>> drivers/gpu/drm/virtio/Kconfig:2:	symbol DRM_VIRTIO_GPU depends on VIRTIO
>>>> drivers/virtio/Kconfig:2:	symbol VIRTIO is selected by GPIO_VIRTIO
>>>> drivers/gpio/Kconfig:1618:	symbol GPIO_VIRTIO depends on GPIOLIB
>>>> drivers/gpio/Kconfig:14:	symbol GPIOLIB is selected by I2C_MUX_LTC4306
>>>> drivers/i2c/muxes/Kconfig:47:	symbol I2C_MUX_LTC4306 depends on I2C
>>>> drivers/i2c/Kconfig:8:	symbol I2C is selected by FB_DDC
>>>> drivers/video/fbdev/Kconfig:63:	symbol FB_DDC depends on FB
>>>> drivers/video/fbdev/Kconfig:12:	symbol FB is selected by DRM_KMS_FB_HELPER
>>>> drivers/gpu/drm/Kconfig:80:	symbol DRM_KMS_FB_HELPER depends on
>>>> DRM_KMS_HELPER
>>>>
>>>> Seems that we can only depend on or select some symbol - we run into
>>>> huge trouble if thats mixed. Just changed DRM_VIRTIO_GPU to just select
>>>> VIRIO instead of depending on it, and now it works.
>>>>
>>>> I've posted another patch for fixing drivers/gpu/drm/virtio/Kconfig
>>>> to use 'select' instead of 'depends on'.
>>> It seems a bit of a mess, at this point I'm not entirely sure when
>>> should drivers select VIRTIO and when depend on it.
>>>
>>> The text near it says:
>>>
>>> # SPDX-License-Identifier: GPL-2.0-only
>>> config VIRTIO
>>>           tristate
>>>           help
>>>             This option is selected by any driver which implements the virtio
>>>             bus, such as CONFIG_VIRTIO_PCI, CONFIG_VIRTIO_MMIO, CONFIG_RPMSG
>>>             or CONFIG_S390_GUEST.
>>>
>>> Which seems clear enough and would indicate drivers for devices *behind*
>>> the bus should not select VIRTIO and thus presumably should "depend on" it.
>>> This is violated in virtio console and virtio fs drivers.
>>>
>>> For console it says:
>>>
>>> commit 9f30eb29c514589e16f2999ea070598583d1f6ec
>>> Author: Michal Suchanek <msuchanek@...e.de>
>>> Date:   Mon Aug 31 18:58:50 2020 +0200
>>>
>>>       char: virtio: Select VIRTIO from VIRTIO_CONSOLE.
>>>       Make it possible to have virtio console built-in when
>>>       other virtio drivers are modular.
>>>       Signed-off-by: Michal Suchanek <msuchanek@...e.de>
>>>       Reviewed-by: Amit Shah <amit@...nel.org>
>>>       Link: https://lore.kernel.org/r/20200831165850.26163-1-msuchanek@suse.de
>>>       Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
>>>
>>> which seems kind of bogus - why do we care about allowing a builtin
>>> virtio console driver if the pci virtio bus driver is a module?
>>> There won't be any devices on the bus to attach to ...
>>
>> For testing like switching bus from pci to MMIO?
>
> Not sure I understand ... can you give an example?


E.g testing

modprobe -r virtio_mmio
modprobe virtio_pci

?


>
>>> And for virtio fs it was like this from the beginning.
>>>
>>> I am inclined to fix console and virtio fs to depend on VIRTIO:
>>> select is harder to use correctly ...
>>>
>>> Jason?
>>
>> I think it works, but we need a prompt for VIRTIO otherwise there's no way
>> to enable it.
>>
>> Thanks
> That's even messier. No one needs VIRTIO core by itself - it's only used
> by transports and drivers.


So we endup with two solutions (without a prompt):

1) using select, user may end up with driver without transport
2) using depends, user need to enable at least one transport

2) looks a little bit better I admit.

Thanks


>
>>>
>>>> -- 
>>>> ---
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>>>> ---
>>>> Enrico Weigelt, metux IT consult
>>>> Free software and Linux embedded engineering
>>>> info@...ux.net -- +49-151-27565287

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