Re: [PATCH v3 10/24] drm/rockchip: dw-mipi-dsi: don't assume buffer is aligned

Sean Paul
2017-01-30T20:08:14Z

On Sun, Jan 29, 2017 at 01:24:30PM +0000, John Keeping wrote:
> By dereferencing the MIPI command buffer as a u32* we rely on it being
> correctly aligned on ARM, but this may not be the case. Copy it into a
> stack variable that will be correctly aligned.
>
> Signed-off-by: John Keeping <john@metanate.com>
> Reviewed-by: Chris Zhong <zyw@rock-chips.com>
> ---
> v3:
> - Add Chris' Reviewed-by
> Unchanged in v2
>
> drivers/gpu/drm/rockchip/dw-mipi-dsi.c | 12 +++++++-----
> 1 file changed, 7 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/gpu/drm/rockchip/dw-mipi-dsi.c b/drivers/gpu/drm/rockchip/dw-mipi-dsi.c
> index 03fc096fe1bd..ddbc037e7ced 100644
> --- a/drivers/gpu/drm/rockchip/dw-mipi-dsi.c
> +++ b/drivers/gpu/drm/rockchip/dw-mipi-dsi.c
> @@ -607,10 +607,10 @@ static int dw_mipi_dsi_dcs_short_write(struct dw_mipi_dsi *dsi,
> static int dw_mipi_dsi_dcs_long_write(struct dw_mipi_dsi *dsi,
> const struct mipi_dsi_msg *msg)
> {
> - const u32 *tx_buf = msg->tx_buf;
> - int len = msg->tx_len, pld_data_bytes = sizeof(*tx_buf), ret;
> + const u8 *tx_buf = msg->tx_buf;
> + int len = msg->tx_len, pld_data_bytes = sizeof(u32), ret;
> u32 hdr_val = GEN_HDATA(msg->tx_len) | GEN_HTYPE(msg->type);
> - u32 remainder = 0;
> + u32 remainder;
> u32 val;
>
> if (msg->tx_len < 3) {
> @@ -621,12 +621,14 @@ static int dw_mipi_dsi_dcs_long_write(struct dw_mipi_dsi *dsi,
>
> while (DIV_ROUND_UP(len, pld_data_bytes)) {
> if (len < pld_data_bytes) {
> + remainder = 0;
> memcpy(&remainder, tx_buf, len);
> dsi_write(dsi, DSI_GEN_PLD_DATA, remainder);
> len = 0;
> } else {
> - dsi_write(dsi, DSI_GEN_PLD_DATA, *tx_buf);
> - tx_buf++;
> + memcpy(&remainder, tx_buf, pld_data_bytes);
> + dsi_write(dsi, DSI_GEN_PLD_DATA, remainder);
> + tx_buf += pld_data_bytes;
> len -= pld_data_bytes;
> }

You can clean this up further by removing a couple of the locals, the
conditional and the division:

while(len < msg->tx_len) {
size_t to_write = MIN(msg->tx_len - len, sizeof(remainder));

memcpy(&remainder, msg->tx_buf + len, to_write);
dsi_write(dsi, DSI_GEN_PLD_DATA, remainder);
len += to_write;

...
}

Sean

>
> --
> 2.11.0.197.gb556de5.dirty
>
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--
Sean Paul, Software Engineer, Google / Chromium OS