Re: [PATCH v2 06/10] drm/ingenic: Set DMA descriptor chain address in probe

From: Paul Cercueil
Date: Tue Jun 30 2020 - 07:55:11 EST


Hi Sam,

Le mar. 30 juin 2020 à 13:44, Sam Ravnborg <sam@xxxxxxxxxxxx> 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 <paul@xxxxxxxxxxxxxxx>
---

Notes:
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.

-Paul

Sam

ret = drm_dev_register(drm, 0);
if (ret) {
dev_err(dev, "Failed to register DRM driver\n");
--
2.27.0

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