linux-kernel - Re: [mtd:nand/next 11/31] drivers/mtd/nand/raw/cadence-nand-controller.c:1893:4: error: implicit declaration of function 'ioread64

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Date:   Thu, 22 Sep 2022 12:52:36 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Miquel Raynal" <miquel.raynal@...tlin.com>,
        "Valentin Korenblit" <vkorenblit@...uans.com>
Cc:     "kernel test robot" <lkp@...el.com>, llvm@...ts.linux.dev,
        kbuild-all@...ts.01.org, linux-kernel@...r.kernel.org
Subject: Re: [mtd:nand/next 11/31]
 drivers/mtd/nand/raw/cadence-nand-controller.c:1893:4: error: implicit
 declaration of function 'ioread64_rep' is invalid in C99

On Thu, Sep 22, 2022, at 11:36 AM, Miquel Raynal wrote:
> vkorenblit@...uans.com wrote on Thu, 22 Sep 2022 10:18:46 +0200:
>> 
>> Correct, this was my initial idea. However, this driver should work
>> with every architecture or do we limit the scope to arm/arm64/x86_64?
>
> The driver should work on ARM and aarch64, I'm not aware of other
> architectures with this IP.
>
> The driver should compile when COMPILE_TEST=y.

It should also be written in a way that makes it plausible to
use elsewhere. Since this is just a licensed IP core, there is
a good chance that someone reused it on mips or riscv, or
anything else.

>> >> I believe what Valentin wanted to achieve in the first place, was to
>> >> use 64-bit accesses when relevant (otherwise it does not work).  
>> > The width is read from a device specific register at
>> > runtime, it is not related to the architecture you are
>> > running on, presumably this is hardwired during the
>> > design of an SoC, based on the capabilities of the DMA
>> > engine:
>
> Well, yes, but in the mean time 64-bit DMA width will never be
> used on 32-bit platforms.

Why? Most architectures (including x86 and arm) allow you to
run a 32-bit kernel on a 64-bit SoC. While this is almost always
a bad idea to actually do, a driver should be written to
work correctly in this setup.

>> > This usually means the largest access that is valid for
>> > reading from the FIFO, but usually smaller accesses work
>> > as well, just slower.  
>
> Mmh, ok, that's interesting, thanks for the pointer.
>
> But in the mean time I am only half satisfied, because we plan to do
> twice more accesses than needed _just_ because of a the COMPILE_TEST
> constraint.

In my example, I had an #ifdef so it would only fall back
to 32-bit accesses on the 64-bit register when running an
actual 32-bit kernel, but leaving the 64-bit case efficient.

    Arnd