Re: [PATCH 03/10] staging:iio:ad7476: Avoid alloc/free for each sample in buffered mode

[Jonathan Cameron <jic23@kernel.org>]

On 09/07/2012 05:37 PM, Lars-Peter Clausen wrote:
> On 09/07/2012 02:44 PM, Lars-Peter Clausen wrote:
>> The ad7476 driver has only support for 1 channel ADCs. So the upper limit for
>> the buffer size is the size of one sample plus the size of the timestamp.
>> Preallocate a buffer large enough to hold this to avoid having to allocate and
>> free a new buffer for each sample being captured.
Sensible move...
>>
>> Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
>> ---
>>  drivers/staging/iio/adc/ad7476.h      |    5 ++++-
>>  drivers/staging/iio/adc/ad7476_ring.c |   13 +++----------
>>  2 files changed, 7 insertions(+), 11 deletions(-)
>>
>> diff --git a/drivers/staging/iio/adc/ad7476.h b/drivers/staging/iio/adc/ad7476.h
>> index 6085fad..c4f1150 100644
>> --- a/drivers/staging/iio/adc/ad7476.h
>> +++ b/drivers/staging/iio/adc/ad7476.h
>> @@ -33,8 +33,11 @@ struct ad7476_state {
>>  	/*
>>  	 * DMA (thus cache coherency maintenance) requires the
>>  	 * transfer buffers to live in their own cache lines.
>> +	 * Make the buffer large enough for one 16 bit sample and one 64 bit
>> +	 * aligned 64 bit timestamp.
>>  	 */
>> -	unsigned char			data[2] ____cacheline_aligned;
>> +	unsigned char data[ALIGN(2, sizeof(s64)) + sizeof(s64)]
>> +			____cacheline_aligned;
>>  };
>>  
>>  enum ad7476_supported_device_ids {
>> diff --git a/drivers/staging/iio/adc/ad7476_ring.c b/drivers/staging/iio/adc/ad7476_ring.c
>> index 940681f..185cfde 100644
>> --- a/drivers/staging/iio/adc/ad7476_ring.c
>> +++ b/drivers/staging/iio/adc/ad7476_ring.c
>> @@ -26,14 +26,9 @@ static irqreturn_t ad7476_trigger_handler(int irq, void  *p)
>>  	struct iio_dev *indio_dev = pf->indio_dev;
>>  	struct ad7476_state *st = iio_priv(indio_dev);
>>  	s64 time_ns;
>> -	__u8 *rxbuf;
>>  	int b_sent;
>>  
>> -	rxbuf = kzalloc(indio_dev->scan_bytes, GFP_KERNEL);
>> -	if (rxbuf == NULL)
>> -		goto done;
>> -
>> -	b_sent = spi_read(st->spi, rxbuf,
>> +	b_sent = spi_read(st->spi, st->data,
>>  			  st->chip_info->channel[0].scan_type.storagebits / 8);
> 
> I just noticed this can even more be simplified by using the prepared SPI
> message. Unfortunately this will require regeneration of a few more patches.
> I'll resend the whole series next week, after it had some exposure to review.

Hmm. Guess that would be a marginal improvement.  Actually from code
readability I'd be tempted to drop the prepared SPI message and
just do the spi_read as you have it here where that message is currently used.

Obviously it'll have some overhead though so up to you which way you go.
> 
>>  	if (b_sent < 0)
>>  		goto done;
>> @@ -41,13 +36,11 @@ static irqreturn_t ad7476_trigger_handler(int irq, void  *p)
>>  	time_ns = iio_get_time_ns();
>>  
>>  	if (indio_dev->scan_timestamp)
>> -		memcpy(rxbuf + indio_dev->scan_bytes - sizeof(s64),
>> -			&time_ns, sizeof(time_ns));
>> +		((s64 *)st->data)[1] = time_ns;
>>  
>> -	iio_push_to_buffer(indio_dev->buffer, rxbuf);
>> +	iio_push_to_buffer(indio_dev->buffer, st->data);
>>  done:
>>  	iio_trigger_notify_done(indio_dev->trig);
>> -	kfree(rxbuf);
>>  
>>  	return IRQ_HANDLED;
>>  }
> 
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