linux-kernel - Re: [PATCH v3] module: create weak dependecies

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Message-ID: <4rfixlve7indkkudtsgyl7bfp5xykzlh3sf2xzlesjbo7xrh44@b4tdzwxxribv>
Date: Tue, 14 May 2024 09:25:55 -0500
From: Lucas De Marchi <lucas.demarchi@...el.com>
To: Jose Ignacio Tornos Martinez <jtornosm@...hat.com>
CC: <mcgrof@...nel.org>, <linux-modules@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] module: create weak dependecies

On Fri, May 10, 2024 at 10:57:22AM GMT, Jose Ignacio Tornos Martinez wrote:
>It has been seen that for some network mac drivers (i.e. lan78xx) the
>related module for the phy is loaded dynamically depending on the current
>hardware. In this case, the associated phy is read using mdio bus and then
>the associated phy module is loaded during runtime (kernel function
>phy_request_driver_module). However, no software dependency is defined, so
>the user tools will no be able to get this dependency. For example, if
>dracut is used and the hardware is present, lan78xx will be included but no
>phy module will be added, and in the next restart the device will not work
>from boot because no related phy will be found during initramfs stage.
>
>In order to solve this, we could define a normal 'pre' software dependency
>in lan78xx module with all the possible phy modules (there may be some),
>but proceeding in that way, all the possible phy modules would be loaded
>while only one is necessary.
>
>The idea is to create a new type of dependency, that we are going to call
>'weak' to be used only by the user tools that need to detect this situation.
>In that way, for example, dracut could check the 'weak' dependency of the
>modules involved in order to install these dependencies in initramfs too.
>That is, for the commented lan78xx module, defining the 'weak' dependency
>with the possible phy modules list, only the necessary phy would be loaded
>on demand keeping the same behavior, but all the possible phy modules would
>be available from initramfs.
>
>The 'weak' dependency support has been included in kmod:
>https://github.com/kmod-project/kmod/commit/05828b4a6e9327a63ef94df544a042b5e9ce4fe7
>But, take into account that this can only be used if depmod is new enough.
>If it isn't, depmod will have the same behavior as always (keeping backward
>compatibility) and the information for the 'weak' dependency will not be
>provided.
>
>Signed-off-by: Jose Ignacio Tornos Martinez <jtornosm@...hat.com>


Reviewed-by: Lucas De Marchi <lucas.demarchi@...el.com>

thanks
Lucas De Marchi

>---
>V2 -> V3:
>- Include note about backward compatibility.
>- Balance the /* and */.
>V1 -> V2:
>- Include reference to 'weak' dependency support in kmod.
>
> include/linux/module.h | 6 ++++++
> 1 file changed, 6 insertions(+)
>
>diff --git a/include/linux/module.h b/include/linux/module.h
>index 1153b0d99a80..2a056017df5b 100644
>--- a/include/linux/module.h
>+++ b/include/linux/module.h
>@@ -173,6 +173,12 @@ extern void cleanup_module(void);
>  */
> #define MODULE_SOFTDEP(_softdep) MODULE_INFO(softdep, _softdep)
>
>+/*
>+ * Weak module dependencies. See man modprobe.d for details.
>+ * Example: MODULE_WEAKDEP("module-foo")
>+ */
>+#define MODULE_WEAKDEP(_weakdep) MODULE_INFO(weakdep, _weakdep)
>+
> /*
>  * MODULE_FILE is used for generating modules.builtin
>  * So, make it no-op when this is being built as a module
>-- 
>2.44.0
>