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

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Date:   Mon, 31 Aug 2020 11:54:58 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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()

Linus Torvalds <torvalds@...ux-foundation.org> writes:
> 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.

The old code is not that old, only ~18 months:

a1ff82a9c165 ("dmaengine: fsldma: Adding macro FSL_DMA_IN/OUT implement for ARM platform") (Jan 2019)

So I think it's possible it's never been tested on 32-bit ppc at all.

I did have a 32-bit FSL machine but it lost its network card in a power
outage and now it won't boot (and I can't get to it physically).

> 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.

Agreed.

Hopefully someone from NXP can test it.

cheers