From 3b56d27ba1578c3d61f51de4102cf896a9a8617e Mon Sep 17 00:00:00 2001 From: Dmitry Baryshkov Date: Thu, 8 Feb 2024 17:23:08 +0200 Subject: [PATCH] drm/msm/dsi: Document DSC related pclk_rate and hdisplay calculations Provide actual documentation for the pclk and hdisplay calculations in the case of DSC compression being used. Signed-off-by: Dmitry Baryshkov Reviewed-by: Abhinav Kumar Patchwork: https://patchwork.freedesktop.org/patch/577534/ Link: https://lore.kernel.org/r/20240208-fd_document_dsc_pclk_rate-v4-1-56fe59d0a2e0@linaro.org --- drivers/gpu/drm/msm/dsi/dsi_host.c | 33 ++++++++++++++++++++++++++++-- 1 file changed, 31 insertions(+), 2 deletions(-) diff --git a/drivers/gpu/drm/msm/dsi/dsi_host.c b/drivers/gpu/drm/msm/dsi/dsi_host.c index 9fa0053fac74f..9d86a6aca6f2a 100644 --- a/drivers/gpu/drm/msm/dsi/dsi_host.c +++ b/drivers/gpu/drm/msm/dsi/dsi_host.c @@ -519,6 +519,25 @@ 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 + * @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 + * + * Porches do not need to be adjusted: + * - For VIDEO mode they are not compressed by DSC and are passed as is. + * - For CMD mode there are no actual porches. Instead these fields + * 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 + * transfer time and data overhead as a starting point of the calculations. + */ static unsigned long dsi_adjust_pclk_for_compression(const struct drm_display_mode *mode, const struct drm_dsc_config *dsc) { @@ -941,8 +960,18 @@ 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 widebus is + * enabled, bus width is extended to 6 bytes. + * + * Calculate the number of pclks needed to transmit one line of + * the compressed data. + + * The back/font porch and pulse width are kept intact. For + * VIDEO mode they represent timing parameters rather than + * actual data transfer, see the documentation for + * dsi_adjust_pclk_for_compression(). For CMD mode they are + * unused anyway. */ h_total -= hdisplay; if (wide_bus_enabled && !(msm_host->mode_flags & MIPI_DSI_MODE_VIDEO))