Re: [PATCH v2 2/2] drivers: gpio: add virtio-gpio guest driver

[Jason Wang]

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
> > > > > > Â Â Â Â Â Â Â  tools/testing/selftests/gpio/gpio-mockup.sh. Reference the
> > > > > > usage in
> > > > > > Â Â Â Â Â Â Â  it.
> > > > > > Â  +config GPIO_VIRTIO
> > > > > > +Â Â Â  tristate "VirtIO GPIO support"
> > > > > > +Â Â Â  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@xxxxxxx>
> > > 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@xxxxxxx>
> > >       Reviewed-by: Amit Shah <amit@xxxxxxxxxx>
> > >       Link: https://lore.kernel.org/r/20200831165850.26163-1-msuchanek@xxxxxxx
> > >       Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > >
> > > 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@xxxxxxxxx -- +49-151-27565287