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

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Date:   Sun, 30 Aug 2020 14:11:54 +0200
From:   Luc Van Oostenryck <luc.vanoostenryck@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        Guenter Roeck <linux@...ck-us.net>,
        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 10:29:55AM -0700, Linus Torvalds wrote:
> On Sat, Aug 29, 2020 at 5:46 AM Luc Van Oostenryck
> <luc.vanoostenryck@...il.com> wrote:
> >
> > But the pointer is already 32-bit, so simply cast the pointer to u32.
> 
> Yeah, that code was completely pointless. If the pointer had actually
> been 64-bit, the old code would have warned too.
> 
> The odd thing is that the fsl_iowrite64() functions make sense. It's
> only the fsl_ioread64() functions that seem to be written by somebody
> who is really confused.
> 
> That said, this patch only humors the confusion. The cast to 'u32' is
> completely pointless. In fact, it seems to be actively wrong, because
> it means that the later "fsl_addr + 1" is done entirely incorrectly -
> it now literally adds "1" to an integer value, while the iowrite()
> functions will add one to a "u32 __iomem *" pointer (so will do
> pointer arithmetic, and add 4).
> 
My bad. I had noticed the '+ 1' and so automatically assumed
'OK, pointer arithmetic now' without noticing that the cast was
done only after the addition. Grrr.

FWIW, the version you committed looks much better to me.

-- Luc