linux-kernel - Re: [PATCH] spi: Remove SPI_BUFSIZ restriction on spi_write_then

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Date:	Thu, 06 Dec 2012 00:00:26 +0000
From:	Grant Likely <grant.likely@...retlab.ca>
To:	Mark Brown <broonie@...nsource.wolfsonmicro.com>
Cc:	spi-devel-general@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>
Subject: Re: [PATCH] spi: Remove SPI_BUFSIZ restriction on spi_write_then_read()

On Sun,  2 Dec 2012 12:54:25 +0900, Mark Brown <broonie@...nsource.wolfsonmicro.com> wrote:
> In order to avoid constantly allocating and deallocating there is a fixed
> buffer which spi_write_then_read() uses for transfers, with an early error
> check to ensure that the transfer fits within the buffer. This limits the
> size of transfers to this size, currently max(32, SMP_CACHE_BYTES).
> 
> Since we can dynamically allocate and in fact already have a fallback
> to do so when there is contention for the fixed buffer remove this
> restriction and instead dynamically allocate a suitably sized buffer if
> the transfer won't fit.
> 
> Signed-off-by: Mark Brown <broonie@...nsource.wolfsonmicro.com>

Looks good to me. Probably 3.9 material though.

Acked-by: Grant Likely <grant.likely@...retlab.ca>

> ---
>  drivers/spi/spi.c |   24 +++++++++++-------------
>  1 file changed, 11 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
> index c4f7d71..224b7bc 100644
> --- a/drivers/spi/spi.c
> +++ b/drivers/spi/spi.c
> @@ -1646,12 +1646,18 @@ int spi_write_then_read(struct spi_device *spi,
>  	struct spi_transfer	x[2];
>  	u8			*local_buf;
>  
> -	/* Use preallocated DMA-safe buffer.  We can't avoid copying here,
> -	 * (as a pure convenience thing), but we can keep heap costs
> -	 * out of the hot path ...
> +	/* Use preallocated DMA-safe buffer if we can.  We can't avoid
> +	 * copying here, (as a pure convenience thing), but we can
> +	 * keep heap costs out of the hot path unless someone else is
> +	 * using the pre-allocated buffer or the transfer is too large.
>  	 */
> -	if ((n_tx + n_rx) > SPI_BUFSIZ)
> -		return -EINVAL;
> +	if ((n_tx + n_rx) > SPI_BUFSIZ || !mutex_trylock(&lock)) {
> +		local_buf = kmalloc(max(SPI_BUFSIZ, n_tx + n_rx), GFP_KERNEL);
> +		if (!local_buf)
> +			return -ENOMEM;
> +	} else {
> +		local_buf = buf;
> +	}
>  
>  	spi_message_init(&message);
>  	memset(x, 0, sizeof x);
> @@ -1664,14 +1670,6 @@ int spi_write_then_read(struct spi_device *spi,
>  		spi_message_add_tail(&x[1], &message);
>  	}
>  
> -	/* ... unless someone else is using the pre-allocated buffer */
> -	if (!mutex_trylock(&lock)) {
> -		local_buf = kmalloc(SPI_BUFSIZ, GFP_KERNEL);
> -		if (!local_buf)
> -			return -ENOMEM;
> -	} else
> -		local_buf = buf;
> -
>  	memcpy(local_buf, txbuf, n_tx);
>  	x[0].tx_buf = local_buf;
>  	x[1].rx_buf = local_buf + n_tx;
> -- 
> 1.7.10.4
> 

-- 
Grant Likely, B.Sc, P.Eng.
Secret Lab Technologies, Ltd.
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