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

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Message-Id: <FFMQCQ.LQ37IHUC6U3Z@crapouillou.net>
Date:   Tue, 30 Jun 2020 13:54:51 +0200
From:   Paul Cercueil <paul@...pouillou.net>
To:     Sam Ravnborg <sam@...nborg.org>
Cc:     David Airlie <airlied@...ux.ie>, Daniel Vetter <daniel@...ll.ch>,
        Rob Herring <robh+dt@...nel.org>, devicetree@...r.kernel.org,
        linux-mips@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, od@...c.me
Subject: Re: [PATCH v2 06/10] drm/ingenic: Set DMA descriptor chain address in
 probe

Hi Sam,

Le mar. 30 juin 2020 à 13:44, Sam Ravnborg <sam@...nborg.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 <paul@...pouillou.net>
>>  ---
>> 
>>  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
>> 
>>  _______________________________________________
>>  dri-devel mailing list
>>  dri-devel@...ts.freedesktop.org
>>  https://lists.freedesktop.org/mailman/listinfo/dri-devel

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