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

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Message-ID: <6e0215da-40f7-128e-5781-00b7d2acbd94@st.com>
Date:   Wed, 11 Oct 2017 17:53:28 +0200
From:   Pierre Yves MORDRET <pierre-yves.mordret@...com>
To:     Arnd Bergmann <arnd@...db.de>,
        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>,
        <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 10/11/2017 05:13 PM, Arnd Bergmann wrote:
> 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.

Both alignment are required on address and length.
The main advantage result is maximized in term of width. As for now I don't see
any drawback except a short explanation.
Nonetheless I need to think a little bit more about this change.

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

Regards
Py