Hi Sam,

Le mar. 30 juin 2020 à 13:44, Sam Ravnborg <s...@ravnborg.org> a écrit :
Hi Paul.

On Tue, Jun 30, 2020 at 01:52:06AM +0200, Paul Cercueil wrote:
The address of the DMA descriptor never changes. It can therefore be set
 in the probe function.

 Signed-off-by: Paul Cercueil <p...@crapouillou.net>

     v2: No change

  drivers/gpu/drm/ingenic/ingenic-drm-drv.c | 5 +++--
  1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/ingenic/ingenic-drm-drv.c b/drivers/gpu/drm/ingenic/ingenic-drm-drv.c
 index 924c8daf071a..f7b0c5dc8cd8 100644
 --- a/drivers/gpu/drm/ingenic/ingenic-drm-drv.c
 +++ b/drivers/gpu/drm/ingenic/ingenic-drm-drv.c
@@ -358,8 +358,6 @@ static void ingenic_drm_crtc_atomic_flush(struct drm_crtc *crtc,
                ingenic_drm_crtc_update_ctrl(priv, finfo);

                clk_set_rate(priv->pix_clk, state->adjusted_mode.clock * 1000);
 -              regmap_write(priv->map, JZ_REG_LCD_DA0, priv->dma_hwdesc->next);

        if (event) {
@@ -757,6 +755,9 @@ static int ingenic_drm_probe(struct platform_device *pdev)

 +      /* Set address of our DMA descriptor chain */
 +      regmap_write(priv->map, JZ_REG_LCD_DA0, priv->dma_hwdesc_phys);

What is the purpose of this code after the assignment was moved:

static int ingenic_drm_probe(struct platform_device *pdev)
        priv->dma_hwdesc->next = priv->dma_hwdesc_phys;

I do not see ->next used anymore.
Could this assignment be dropped?

And the following line:
        priv->dma_hwdesc->id = 0xdeafbead;

I can only see ->id be assingned, it is never referenced??

Maybe this is all related to the structure assumed by the data pointed
to by JZ_REG_LCD_DA0?

As its name suggests, 'dma_hwdesc' points to a DMA hardware descriptor. Setting ->next sets the address of the next descriptor (which is itself since we only use one descriptor). The 'id' field will be copied by the hardware to a register when the DMA descriptor is loaded, so we can know from userspace (debugfs) what DMA descriptors are loaded by looking up the IDs.



        ret = drm_dev_register(drm, 0);
        if (ret) {
                dev_err(dev, "Failed to register DRM driver\n");

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