Re: [PATCH v8 24/25] powerpc: Adopt nvram module for PPC64

On Sun, Dec 30, 2018 at 4:29 AM Finn Thain [off-list ref] wrote:

On Sat, 29 Dec 2018, Arnd Bergmann wrote:

quoted

On Wed, Dec 26, 2018 at 1:43 AM Finn Thain [off-list ref] wrote:

quoted

+static ssize_t ppc_nvram_get_size(void)
+{
+       if (ppc_md.nvram_size)
+               return ppc_md.nvram_size();
+       return -ENODEV;
+}

quoted

+const struct nvram_ops arch_nvram_ops = {
+       .read           = ppc_nvram_read,
+       .write          = ppc_nvram_write,
+       .get_size       = ppc_nvram_get_size,
+       .sync           = ppc_nvram_sync,
+};

Coming back to this after my comment on the m68k side, I notice that
there is now a double indirection through function pointers. Have you
considered completely removing the operations from ppc_md instead by
having multiple copies of nvram_ops?

I considered a few alternatives. I figured that it was refactoring that
could be deferred, as it would be confined to arch/powerpc. I was more
interested in the cross-platform API.

Fair enough.

quoted

With the current method, it does seem odd to have a single
per-architecture instance of the exported structure containing function
pointers. This doesn't give us the flexibility of having multiple copies
in the kernel the way that ppc_md does, but it adds overhead compared to
simply exporting the functions directly.

You're right, there is overhead here.

With a bit of auditing, wrappers like the one you quoted (which merely
checks whether or not a ppc_md method is implemented) could surely be
avoided.

The arch_nvram_ops methods are supposed to optional (that is, they are
allowed to be NULL).

We could call exactly the same function pointers though either ppc_md or
arch_nvram_ops. That would avoid the double indirection.

I think you can have a 'const' structure in the __ro_after_init section,
so without changing anything else, powerpc could just copy the
function pointers from ppc_md into the arch_nvram_ops at early
init time, which should ideally simplify your implementation as well.

        Arnd