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

[Emil Velikov]

On Tue, 5 May 2020 at 17:05, Emil Velikov <emil.l.velikov at gmail.com> 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>
> ---
>  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;
> --

Thierry, others - humble ping.
Can you check through the series?

Thanks
Emil