RE: [PATCH] fsldma: fsl_ioread64*() do not need lower_32_bits()

[Leo Li <leoyang.li@nxp.com>]

> -----Original Message-----
> From: Linus Torvalds <torvalds@linux-foundation.org>
> Sent: Saturday, August 29, 2020 4:20 PM
> To: Guenter Roeck <linux@roeck-us.net>
> Cc: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>; Herbert Xu
> <herbert@gondor.apana.org.au>; Andrew Morton <akpm@linux-
> foundation.org>; Joerg Roedel <joerg.roedel@amd.com>; Leo Li
> <leoyang.li@nxp.com>; Zhang Wei <zw@zh-kernel.org>; Dan Williams
> <dan.j.williams@intel.com>; Vinod Koul <vkoul@kernel.org>; linuxppc-dev
> <linuxppc-dev@lists.ozlabs.org>; dma <dmaengine@vger.kernel.org>; Linux
> Kernel Mailing List <linux-kernel@vger.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@roeck-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@roeck-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.

Thanks for the patch.  

Acked-by: Li Yang <leoyang.li@nxp.com>

We are having periodical auto regression tests covering ppc32 platforms.  But looks like it missed this issue.  I will ask the test team to investigate on why the test cases are not sufficient.

Regards,
Leo