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

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Date:   Mon, 31 Aug 2020 12:09:06 +0530
From:   Vinod Koul <vkoul@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Guenter Roeck <linux@...ck-us.net>,
        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>,
        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()

Hi Linus,

On 29-08-20, 14:20, Linus Torvalds wrote:
> 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.

Thank you for the fix.

Acked-By: Vinod Koul <vkoul@...nel.org>

> 
> 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.

I guess no one tested this on 32bits seems to have caused this.

-- 
~Vinod