linux-kernel - Re: [PATCH] dmaengine: stm32-mdma: avoid 64-bit division

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Message-ID: <CAK8P3a3j5Awh9V6+PHZf-79TM_7wdQ0GkZZv75a3ztewrw6CmQ@mail.gmail.com>
Date:   Wed, 11 Oct 2017 17:13:25 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Benjamin Gaignard <benjamin.gaignard@...aro.org>
Cc:     Vinod Koul <vinod.koul@...el.com>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Alexandre Torgue <alexandre.torgue@...com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Pierre-Yves MORDRET <pierre-yves.mordret@...com>,
        dmaengine@...r.kernel.org,
        "M'boumba Cedric Madianga" <cedric.madianga@...il.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] dmaengine: stm32-mdma: avoid 64-bit division

On Wed, Oct 11, 2017 at 4:46 PM, Benjamin Gaignard
<benjamin.gaignard@...aro.org> wrote:
> 2017-10-11 16:39 GMT+02:00 Arnd Bergmann <arnd@...db.de>:
>> On Wed, Oct 11, 2017 at 4:27 PM, Benjamin Gaignard
>> <benjamin.gaignard@...aro.org> wrote:
>>> 2017-10-11 16:01 GMT+02:00 Arnd Bergmann <arnd@...db.de>:
>>>
>>>> @@ -398,6 +400,9 @@ static enum dma_slave_buswidth stm32_mdma_get_max_width(u32 buf_len, u32 tlen)
>>>>                         break;
>>>>         }
>>>>
>>>> +       if (addr % max_width)
>>>> +               max_width = DMA_SLAVE_BUSWIDTH_1_BYTE;
>>>> +
>>>
>>> I'm only half-convince by the implicite 32 bits cast done into
>>> function prototype.
>>> If we keep using dma_addr_t and use do_div() instead of %
>>> does compiler can still optimize the code ?
>>>
>>
>> I wouldn't want to add a do_div() here, since it's guaranteed
>> not to be needed. Would you prefer an explicit cast here
>> and leave the argument as dma_addr_t?
>>
>> We could also use a bit mask here like
>>
>>   if (addr & (max_width-1))
>
> That sound better for me since it doesn't limit the code to 32 bits architecture

FWIW, I used the u32 type here because that's the limit of the
dma driver, the dma_addr_t gets converted to that anyway
later.

>>
>> or we could combined it with the check above:
>>
>>                 if ((((buf_len | addr) & (max_width - 1)) == 0) &&
>>                    (tlen >= max_width))
>
> No it is more simple to read with two checks

I should have mentioned that this variant would also change
behavior: the current code falls back to byte access when
the address alignment is less than the length alignment.
The change I suggested here would change that to use
the maximum possible address width that fits the alignment
of either size or address.

I don't know what behavior we actually want though, or
if that change would be correct.

      Arnd