linux-kernel - Re: [PATCH] fsldma: fsl_ioread64*() do not need lower_32

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Date:   Sat, 29 Aug 2020 14:20:14 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Joerg Roedel <joerg.roedel@....com>,
        Li Yang <leoyang.li@....com>, Zhang Wei <zw@...kernel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Vinod Koul <vkoul@...nel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        dma <dmaengine@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] fsldma: fsl_ioread64*() do not need lower_32_bits()

On Sat, Aug 29, 2020 at 1:40 PM Guenter Roeck <linux@...ck-us.net> wrote:
>
> Except for
>
> CHECK: spaces preferred around that '+' (ctx:VxV)
> #29: FILE: drivers/dma/fsldma.h:223:
> +       u32 val_lo = in_be32((u32 __iomem *)addr+1);

Added spaces.

> I don't see anything wrong with it either, so
>
> Reviewed-by: Guenter Roeck <linux@...ck-us.net>
>
> Since I didn't see the real problem with the original code,
> I'd take that with a grain of salt, though.

Well, honestly, the old code was so confused that just making it build
is clearly already an improvement even if everything else were to be
wrong.

So I committed my "fix". If it turns out there's more wrong in there
and somebody tests it, we can fix it again. But now it hopefully
compiles, at least.

My bet is that if that driver ever worked on ppc32, it will continue
to work whatever we do to that function.

I _think_ the old code happened to - completely by mistake - get the
value right for the case of "little endian access, with dma_addr_t
being 32-bit". Because then it would still read the upper bits wrong,
but the cast to dma_addr_t would then throw those bits away. And the
lower bits would be right.

But for big-endian accesses or for ARCH_DMA_ADDR_T_64BIT it really
looks like it always returned a completely incorrect value.

And again - the driver may have worked even with that completely
incorrect value, since the use of it seems to be very incidental.

In either case ("it didn't work before" or "it worked because the
value doesn't really matter"), I don't think I could possibly have
made things worse.

Famous last words.

                Linus