[PATCH 1/3] drm/dsi: use stack buffer in mipi_dsi_dcs_write()

[Sam Ravnborg]

On Tue, May 05, 2020 at 05:03:27PM +0100, Emil Velikov wrote:
> Currently the function heap allocates when we have any payload. Where in
> many case the payload is 1 byte - ouch.
> 
> >From casual observation, vast majority of the payloads are smaller than
> 8 bytes - so use a stack array tx[8] to avoid the senseless kmalloc and
> kfree dance.
> 
> Cc: Jani Nikula <jani.nikula at intel.com>
> Cc: Thierry Reding <treding at nvidia.com>
> Signed-off-by: Emil Velikov <emil.velikov at collabora.com>

Applied to drm-misc-next. Sorry for taking so long.

	Sam

> ---
>  drivers/gpu/drm/drm_mipi_dsi.c | 20 ++++++++++----------
>  1 file changed, 10 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_mipi_dsi.c b/drivers/gpu/drm/drm_mipi_dsi.c
> index 55531895dde6..b96d5b4629d7 100644
> --- a/drivers/gpu/drm/drm_mipi_dsi.c
> +++ b/drivers/gpu/drm/drm_mipi_dsi.c
> @@ -748,26 +748,26 @@ ssize_t mipi_dsi_dcs_write(struct mipi_dsi_device *dsi, u8 cmd,
>  {
>  	ssize_t err;
>  	size_t size;
> +	u8 stack_tx[8];
>  	u8 *tx;
>  
> -	if (len > 0) {
> -		size = 1 + len;
> -
> +	size = 1 + len;
> +	if (len > ARRAY_SIZE(stack_tx) - 1) {
>  		tx = kmalloc(size, GFP_KERNEL);
>  		if (!tx)
>  			return -ENOMEM;
> -
> -		/* concatenate the DCS command byte and the payload */
> -		tx[0] = cmd;
> -		memcpy(&tx[1], data, len);
>  	} else {
> -		tx = &cmd;
> -		size = 1;
> +		tx = stack_tx;
>  	}
>  
> +	/* concatenate the DCS command byte and the payload */
> +	tx[0] = cmd;
> +	if (data)
> +		memcpy(&tx[1], data, len);
> +
>  	err = mipi_dsi_dcs_write_buffer(dsi, tx, size);
>  
> -	if (len > 0)
> +	if (tx != stack_tx)
>  		kfree(tx);
>  
>  	return err;
> -- 
> 2.25.1
> 
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