Re: [PATCH v3 24/24] drm/rockchip: dw-mipi-dsi: support read commands

John Keeping
2017-01-30T18:14:27Z

On Mon, 30 Jan 2017 10:26:11 -0500, Sean Paul wrote:

> On Sun, Jan 29, 2017 at 01:24:44PM +0000, John Keeping wrote:
> > I haven't found any method for getting the length of a response, so this
> > just uses the requested rx_len
> >
> > Signed-off-by: John Keeping <john@metanate.com>
> > ---
> > v3:
> > - Fix checkpatch warnings
> > Unchanged in v2
> >
> > drivers/gpu/drm/rockchip/dw-mipi-dsi.c | 56 ++++++++++++++++++++++++++++++++++
> > 1 file changed, 56 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/rockchip/dw-mipi-dsi.c b/drivers/gpu/drm/rockchip/dw-mipi-dsi.c
> > index cf3ca6b0cbdb..cc58ada75425 100644
> > --- a/drivers/gpu/drm/rockchip/dw-mipi-dsi.c
> > +++ b/drivers/gpu/drm/rockchip/dw-mipi-dsi.c
> > @@ -678,6 +678,56 @@ static int dw_mipi_dsi_dcs_long_write(struct dw_mipi_dsi *dsi,
> > return dw_mipi_dsi_gen_pkt_hdr_write(dsi, hdr_val);
> > }
> >
> > +static int dw_mipi_dsi_dcs_read(struct dw_mipi_dsi *dsi,
> > + const struct mipi_dsi_msg *msg)
> > +{
> > + const u8 *tx_buf = msg->tx_buf;
> > + u8 *rx_buf = msg->rx_buf;
> > + size_t i;
> > + int ret, val;
> > +
> > + dsi_write(dsi, DSI_PCKHDL_CFG, EN_CRC_RX | EN_ECC_RX | EN_BTA);
> > + dsi_write(dsi, DSI_GEN_HDR,
> > + GEN_HDATA(tx_buf[0]) | GEN_HTYPE(msg->type));
> > +
> > + ret = readl_poll_timeout(dsi->base + DSI_CMD_PKT_STATUS,
> > + val, !(val & GEN_RD_CMD_BUSY), 1000,
> > + CMD_PKT_STATUS_TIMEOUT_US);
> > + if (ret < 0) {
> > + dev_err(dsi->dev, "failed to read command response\n");
> > + return ret;
> > + }
> > +
> > + for (i = 0; i < msg->rx_len;) {
> > + u32 pld = dsi_read(dsi, DSI_GEN_PLD_DATA);
> > +
> > + while (i < msg->rx_len) {
> > + rx_buf[i] = pld & 0xff;
> > + pld >>= 8;
> > + i++;
> > + }
> > + }
>
> AFAICT, the outer for loop just initializes i and ensures msg->rx_len is
> non-zero?
>
> I think the following would be easier to read (and safe against the case where
> msg->rx_len > sizeof(pld) (even though this shouldn't happen according to DCS
> spec)).
>
> if (msg->rx_len > 0) {
> u32 pld = dsi_read(dsi, DSI_GEN_PLD_DATA);
> memcpy(rx_buf, &pld, MIN(msg->rx_len, sizeof(pld));
> }

I think the intent was to handle rx_len > 4, but the patch is obvously
completely broken regarding that. As far as I can tell, rx_len is
limited by the maximum return packet size which can be any value up to
the maximum size of a long packet, so we may need to read from the FIFO
multiple times.

The loop should be something like this:

for (i = 0; i < msg->rx_len;) {
u32 pld = dsi_read(dsi, DSI_GEN_PLD_DATA);
int j;

for (j = 0; j < 4 && i < msg->rx_len; i++, j++) {
rx_buf[i] = pld & 0xff;
pld >>= 8;
}
}

I have successfully read 5 bytes from a DSI display using this code, but
I'm tempted to just drop this patch since I only used it for debugging
while bringing up a new panel.