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

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Message-ID: <59cc6c99-9894-08b3-1075-2156e39bfc8e@roeck-us.net>
Date:   Sat, 29 Aug 2020 13:40:07 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>
Cc:     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 8/29/20 10:29 AM, 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).
> 

Outch.

> So this code has never ever worked correctly to begin with, but the
> patches to fix the warning miss the point. The problem isn't the
> warning, the problem is that the code is broken and completely wrong
> to begin with.
> 
> And the "lower_32_bits()" thing has always been pure and utter
> confusion and complete garbage.
> 
> I *think* the right patch is the one attached, but since this code is
> clearly utterly broken, I'd want somebody to test it.
> 
> It has probably never ever worked on 32-bit powerpc, or did so purely
> by mistake (perhaps because nobody really cares - the only 64-bit use
> is this:
> 
>     static dma_addr_t get_cdar(struct fsldma_chan *chan)
>     {
>         return FSL_DMA_IN(chan, &chan->regs->cdar, 64) & ~FSL_DMA_SNEN;
>     }
> 
> and there are two users of that: one which ignores the return value,
> and one that looks like it might end up half-way working even if the
> value read was garbage (it's used only to compare against a "current
> descriptor" value).
> 
> Anyway, the fix is definitely not to just shut up the warning. The
> warning is only a sign of utter confusion in that driver.
> 
> Can somebody with the hardware test this on 32-bit ppc?
> 
> And if not (judging by just how broken those functions are, maybe it
> never did work), can somebody with a ppc32 setup at least compile-test
> this patch and look at whether it makes sense, in ways the old code
> did not.
> 

A bit more careful this time. For the attached patch:

Compile-tested-by: Guenter Roeck <linux@...ck-us.net>

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);

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.

Guenter