On 20/06/2023 15:05, Marijn Suijten wrote:
On 2023-06-20 00:06:47, Dmitry Baryshkov wrote:
Provide actual documentation for the pclk and hdisplay calculations in
the case of DSC compression being used.
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@xxxxxxxxxx>
---
Changes since v1:
- Converted dsi_adjust_pclk_for_compression() into kerneldoc (Marijn)
- Added a pointer from dsi_timing_setup() docs to
dsi_adjust_pclk_for_compression() (Marijn)
- Fixed two typo (Marijn)
---
drivers/gpu/drm/msm/dsi/dsi_host.c | 40 ++++++++++++++++++++++++++++--
1 file changed, 38 insertions(+), 2 deletions(-)
diff --git a/drivers/gpu/drm/msm/dsi/dsi_host.c b/drivers/gpu/drm/msm/dsi/dsi_host.c
index 3f6dfb4f9d5a..a8a31c3dd168 100644
--- a/drivers/gpu/drm/msm/dsi/dsi_host.c
+++ b/drivers/gpu/drm/msm/dsi/dsi_host.c
@@ -528,6 +528,25 @@ void dsi_link_clk_disable_v2(struct msm_dsi_host *msm_host)
clk_disable_unprepare(msm_host->byte_clk);
}
+/**
+ * dsi_adjust_pclk_for_compression() - Adjust the pclk rate for compression case
+ * @mode: the selected mode for the DSI output
The
+ * @dsc: DRM DSC configuration for this DSI output
+ *
+ * Adjust the pclk rate by calculating a new hdisplay proportional to
+ * the compression ratio such that:
+ * new_hdisplay = old_hdisplay * compressed_bpp / uncompressed_bpp
And in v1 you explained that it is _not_ about bpp...
Hmm, this bit stayed intact, so I'm slightly confused here.
This function is about BPP and compressed rate. dsi_timing_setup() is
about bytes.
+ *
+ * Porches do not need to be adjusted:
+ * - For the VIDEO mode they are not compressed by DSC and are passed as is.
+ * - For the CMD mode there are no actual porches. Instead these fields
I feel like "For VIDEO mode" and "For CMD mode" reads more naturally, no
need for "the", but I don't know the grammar rule that states so.
Ack
+ * currently represent the overhead to the image data transfer. As such, they
+ * are calculated for the final mode parameters (after the compression) and
+ * are not to be adjusted too.
+ *
+ * FIXME: Reconsider this if/when CMD mode handling is rewritten to use
+ * refresh rate and data overhead as a starting point of the calculations.
Nit: well, refresh rate is already part of this calculation (that's how
drm_display_mode's clock member comes to be, and how drm_mode_vrefresh()
figures out fps after the fact). It's all about the per-CMD transfer
overhead in bytes that is currently not part of the calculation.
Please correct me if I'm wrong, we start from mode->clock. Refresh rate
isn't even a part of struct drm_display_mode.
+ */
static unsigned long dsi_adjust_pclk_for_compression(const struct drm_display_mode *mode,
const struct drm_dsc_config *dsc)
{
@@ -926,8 +945,25 @@ static void dsi_timing_setup(struct msm_dsi_host *msm_host, bool is_bonded_dsi)
if (ret)
return;
- /* Divide the display by 3 but keep back/font porch and
- * pulse width same
+ /*
+ * DPU sends 3 bytes per pclk cycle to DSI. If compression is
Should this be pixels (1 pixel), not bytes, just like in the compressed
scenario?
No.
+ * not used, a single pixel is transferred at each pulse, no
+ * matter what bpp or pixel format is used. In case of DSC
+ * compression this results (due to data alignment
+ * requirements) in a transfer of 3 compressed pixel per pclk
+ * cycle.
+ *
+ * If widebus is enabled, bus width is extended to 6 bytes.
+ * This way the DPU can transfer 6 compressed pixels with bpp
+ * less or equal to 8 or 3 compressed pixels in case bpp is
+ * greater than 8.
Okay, so one can not send more than 6 pixels even if the bpp is less
than 8, is what this comes down to.
Yes.
+ *
+ * The back/font porch and pulse width are kept intact. They
+ * represent timing parameters rather than actual data
+ * transfer. See the documentation of
+ * dsi_adjust_pclk_for_compression().
Nit: note that this is only for VIDEO mode, h_total and ha_end are
accurately unused in the CMDmode path below.
- Marijn
+ *
+ * XXX: widebus is not supported by the driver (yet).
*/
h_total -= hdisplay;
hdisplay = DIV_ROUND_UP(msm_dsc_get_bytes_per_line(msm_host->dsc), 3);
--
2.39.2
--
With best wishes
Dmitry
