linux-kernel - Re: [PATCH v2 1/2] dmaengine: stm32-mdma: align TLEN and buffer length on burst

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Date:   Fri, 13 Apr 2018 11:45:44 +0200
From:   Pierre Yves MORDRET <pierre-yves.mordret@...com>
To:     Robin Murphy <robin.murphy@....com>,
        Vinod Koul <vinod.koul@...el.com>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Alexandre Torgue <alexandre.torgue@...com>,
        Dan Williams <dan.j.williams@...el.com>,
        "M'boumba Cedric Madianga" <cedric.madianga@...il.com>,
        <dmaengine@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] dmaengine: stm32-mdma: align TLEN and buffer
 length on burst

Hi Robin

On 04/11/2018 05:14 PM, Robin Murphy wrote:
> On 11/04/18 15:44, Pierre-Yves MORDRET wrote:
>> Both buffer Transfer Length (TLEN if any) and transfer size have to be
>> aligned on burst size (burst beats*bus width).
>>
>> Signed-off-by: Pierre-Yves MORDRET <pierre-yves.mordret@...com>
>> ---
>>    Version history:
>>      v1:
>>         * Initial
>>      v2:
>> ---
>> ---
>>   drivers/dma/stm32-mdma.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/dma/stm32-mdma.c b/drivers/dma/stm32-mdma.c
>> index daa1602..fbcffa2 100644
>> --- a/drivers/dma/stm32-mdma.c
>> +++ b/drivers/dma/stm32-mdma.c
>> @@ -413,7 +413,7 @@ static u32 stm32_mdma_get_best_burst(u32 buf_len, u32 tlen, u32 max_burst,
>>   	u32 best_burst = max_burst;
>>   	u32 burst_len = best_burst * width;
>>   
>> -	while ((burst_len > 0) && (tlen % burst_len)) {
>> +	while ((burst_len > 0) && (((tlen | buf_len) & (burst_len - 1)) != 0)) {
>>   		best_burst = best_burst >> 1;
>>   		burst_len = best_burst * width;
>>   	}
> 
> FWIW, doesn't that whole loop come down to just:
> 
> 	burst_len = min(ffs(tlen | buf_len), max_burst * width);

No sure it ends as expected. or I miss something or don't understand this statement
I tried with "relevant value" : i.e. best_burst = 32, Tlen=128(default) and
buf_len = 64, width= 4. This statements gets me something wrong output => 7
instead of 16 * 4.
I doubt :)

> 
> ?
> 
> Robin.
>