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Date: Thu, 9 May 2024 08:06:34 -0500
From: Lucas De Marchi <lucas.demarchi@intel.com>
To: Jose Ignacio Tornos Martinez <jtornosm@redhat.com>
CC: <mcgrof@kernel.org>, <linux-modules@vger.kernel.org>,
	<linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2] module: create weak dependecies
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References: <20240509102442.176958-1-jtornosm@redhat.com>
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On Thu, May 09, 2024 at 12:24:40PM 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.

I think it's important a note about backward compatibility. If a system
doesn't have a new-enough depmod, it will basically not create the new
weadep file and initrd generators won't be able to use that. Only
downside is not being able to use the new feature, but it should still
work as previously.

>
>The 'weak' dependency support has been included in kmod:
>https://github.com/kmod-project/kmod/commit/05828b4a6e9327a63ef94df544a042b5e9ce4fe7
>
>Signed-off-by: Jose Ignacio Tornos Martinez <jtornosm@redhat.com>
>---
>V1 -> V2:
>- Include reference to 'weak' dependency support in kmod.
>
> include/linux/module.h | 5 +++++
> 1 file changed, 5 insertions(+)
>
>diff --git a/include/linux/module.h b/include/linux/module.h
>index 1153b0d99a80..231e710d8736 100644
>--- a/include/linux/module.h
>+++ b/include/linux/module.h
>@@ -173,6 +173,11 @@ extern void cleanup_module(void);
>  */
> #define MODULE_SOFTDEP(_softdep) MODULE_INFO(softdep, _softdep)
>
>+/* Weak module dependencies. See man modprobe.d for details.

Documentation/process/coding-style.rst section 8 says to balance the /*
and */, but up to Luis what he enforces in the module tree.

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

Lucas De Marchi

>+ * 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
>