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

[Paul Cercueil]

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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