Re: [Freedreno] [PATCH v5 2/5] drm/msm/dsi: Adjust pclk rate for compression

From: Jessica Zhang
Date: Thu Jun 08 2023 - 20:57:26 EST



On 6/8/2023 1:36 PM, Marijn Suijten wrote:
Same title suggestion as earlier: s/adjust/reduce

Hi Marijn,

Acked.


On 2023-05-22 18:08:56, Jessica Zhang wrote:
Adjust the pclk rate to divide hdisplay by the compression ratio when DSC
is enabled.

Signed-off-by: Jessica Zhang <quic_jesszhan@xxxxxxxxxxx>
---
drivers/gpu/drm/msm/dsi/dsi_host.c | 21 ++++++++++++++++++---
1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/msm/dsi/dsi_host.c b/drivers/gpu/drm/msm/dsi/dsi_host.c
index a448931af804..88f370dd2ea1 100644
--- a/drivers/gpu/drm/msm/dsi/dsi_host.c
+++ b/drivers/gpu/drm/msm/dsi/dsi_host.c
@@ -561,7 +561,18 @@ void dsi_link_clk_disable_v2(struct msm_dsi_host *msm_host)
clk_disable_unprepare(msm_host->byte_clk);
}
-static unsigned long dsi_get_pclk_rate(const struct drm_display_mode *mode, bool is_bonded_dsi)
+static unsigned long dsi_adjust_compressed_pclk(const struct drm_display_mode *mode,

Nit: adjust_pclk_for_compression

Acked.


As discussed before we realized that this change is more-or-less a hack,
since downstream calculates pclk quite differently - at least for
command-mode panels. Do you still intend to land this patch this way,
or go the proper route by introducing the right math from the get-go?
Or is the math at least correct for video-mode panels?

Sorry but can you please clarify what exactly is incorrect or different about this math when compared to downstream? And, if you think that this math is incorrect, what exactly has to be changed to make it the "right math"?

We've already shown step-by-step [1] not only how the resulting pclk from the downstream code matches out upstream calculations, but also how the inclusion of porches in the upstream math would make up for the fact that upstream has no concept of transfer time [2].

If the lack of transfer time in the upstream math is the issue, I believe that that's not within the scope of this series, as transfer time is not something specific to DSC.

Moreover, as stated in an earlier revision [3], there is no way to validate DSC over DSI for video mode. As far as I know, we do not have a way to validate video mode datapath for DSI in general.

[1] https://gitlab.freedesktop.org/drm/msm/-/issues/24#note_1936144
[2] https://gitlab.freedesktop.org/drm/msm/-/issues/24#note_1945792
[3] https://patchwork.freedesktop.org/patch/535117/?series=117219&rev=1#comment_970492


This function requires a documentation comment to explain that all.

+ const struct drm_dsc_config *dsc)
+{
+ int new_hdisplay = DIV_ROUND_UP(mode->hdisplay * drm_dsc_get_bpp_int(dsc),

This sounds like a prime candidate for msm_dsc_get_bytes_per_line(), if
bits_per_component==8 is assumed. In fact, it then becomes identical
to the following line in dsi_host.c which you added previously:

hdisplay = DIV_ROUND_UP(msm_dsc_get_bytes_per_line(msm_host->dsc), 3);

If not, what is the difference between these two calculations? Maybe
they both need to be in a properly-named helper.

While the math technically matches up (assuming, also, that mode->hdisplay == slice_width * slice_count for all cases), there are conceptual differences between the pclk and hdisplay calculations.

Just to reiterate what was already said on IRC:

In the pclk calculation, we're multiplying pclk by the compression ratio (which would be target_bpp / src_bpp) -- please note that here, we calculate src_bpp by doing bpc * 3.

In the hdisplay calculation, we calculate the bytes per line and divide by 3 (bytes) to account for the fact that we can only process 3 bytes per pclk cycle.

So while I understand why you would want to put this behind a shared helper, I think abstracting the pclk and hdisplay math away would obfuscate the conceptual difference between the 2 calculations.


+ dsc->bits_per_component * 3);

As we established in the drm/msm issue [2] there is currently a
confusion whether this /3 (and the /3 in dsi_timing_setup) come from the
ratio between dsi_get_bpp() and dsc->bpp or something else. Can you
clarify that with constants and comments?

Sure, we are planning to add a patch to the end of this series documenting the math.


[2]: https://gitlab.freedesktop.org/drm/msm/-/issues/24

+
+ return (new_hdisplay + (mode->htotal - mode->hdisplay))
+ * mode->vtotal * drm_mode_vrefresh(mode);

As clarified in [1] I was not necessarily suggesting to move this math
to a separate helper, but to also use a few more properly-named
intermediate variables to not have multi-line math and self-documenting
code. These lines could be split to be much more clear.

Acked.


[1]: https://lore.kernel.org/linux-arm-msm/u4x2vldkzsokfcpbkz3dtwcllbdk4ljcz6kzuaxt5frx6g76o5@uku6abewvye7/

+}
+
+static unsigned long dsi_get_pclk_rate(const struct drm_display_mode *mode,
+ const struct drm_dsc_config *dsc, bool is_bonded_dsi)
{
unsigned long pclk_rate;
@@ -576,6 +587,10 @@ static unsigned long dsi_get_pclk_rate(const struct drm_display_mode *mode, bool
if (is_bonded_dsi)
pclk_rate /= 2;
+ /* If DSC is enabled, divide hdisplay by compression ratio */
+ if (dsc)
+ pclk_rate = dsi_adjust_compressed_pclk(mode, dsc);

The confusion with this comment (and the reason the aforementioned
discussion [2] carried on so long) stems from the fact a division makes
sense for a bit/byte clock, but not for a pixel clock: we still intend
to send the same number of pixels, just spending less bytes on them. So
as you clarify the /3 above, can you also clarify that here or drop this
comment completely when the function is correctly documented instead?

Acked.

Thanks,

Jessica Zhang


- Marijn

+
return pclk_rate;
}
@@ -585,7 +600,7 @@ unsigned long dsi_byte_clk_get_rate(struct mipi_dsi_host *host, bool is_bonded_d
struct msm_dsi_host *msm_host = to_msm_dsi_host(host);
u8 lanes = msm_host->lanes;
u32 bpp = dsi_get_bpp(msm_host->format);
- unsigned long pclk_rate = dsi_get_pclk_rate(mode, is_bonded_dsi);
+ unsigned long pclk_rate = dsi_get_pclk_rate(mode, msm_host->dsc, is_bonded_dsi);
unsigned long pclk_bpp;
if (lanes == 0) {
@@ -604,7 +619,7 @@ unsigned long dsi_byte_clk_get_rate(struct mipi_dsi_host *host, bool is_bonded_d
static void dsi_calc_pclk(struct msm_dsi_host *msm_host, bool is_bonded_dsi)
{
- msm_host->pixel_clk_rate = dsi_get_pclk_rate(msm_host->mode, is_bonded_dsi);
+ msm_host->pixel_clk_rate = dsi_get_pclk_rate(msm_host->mode, msm_host->dsc, is_bonded_dsi);
msm_host->byte_clk_rate = dsi_byte_clk_get_rate(&msm_host->base, is_bonded_dsi,
msm_host->mode);

--
2.40.1