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

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Message-ID: <20201208101051.GC6564@kitsune.suse.cz>
Date:   Tue, 8 Dec 2020 11:10:51 +0100
From:   Michal Suchánek <msuchanek@...e.de>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     "Enrico Weigelt, metux IT consult" <lkml@...ux.net>,
        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
Subject: Re: [PATCH v2 2/2] drivers: gpio: add virtio-gpio guest driver

Hello

On Sat, Dec 05, 2020 at 02:32:04PM -0500, 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:
> > 
> > > 
> > > 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 ...
The console driver provides early console which should initialize
without any transport. I have not tested it actually works but the code
seems to be there to support this use case.

Thanks

Michal