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

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Message-ID: <20201205142218-mutt-send-email-mst@kernel.org>
Date:   Sat, 5 Dec 2020 14:32:04 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     "Enrico Weigelt, metux IT consult" <lkml@...ux.net>
Cc:     Jason Wang <jasowang@...hat.com>,
        "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 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 ...

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?


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