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

["Michael S. Tsirkin" <mst@redhat.com>]

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@suse.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@suse.de>
> >      Reviewed-by: Amit Shah <amit@kernel.org>
> >      Link: https://lore.kernel.org/r/20200831165850.26163-1-msuchanek@suse.de
> >      Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.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?

> 
> > 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.

> 
> > 
> > 
> > > -- 
> > > ---
> > > Hinweis: unverschlüsselte E-Mails können leicht abgehört und manipuliert
> > > werden ! Für eine vertrauliche Kommunikation senden Sie bitte ihren
> > > GPG/PGP-Schlüssel zu.
> > > ---
> > > Enrico Weigelt, metux IT consult
> > > Free software and Linux embedded engineering
> > > info@metux.net -- +49-151-27565287