linux-kernel - Re: [PATCH v7 13/20] drm/mediatek: Add programming flow for unsupported subsys ID hardware

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Message-ID: <6690e20f-f88e-4c5a-8188-4d2a941fc6b1@collabora.com>
Date: Thu, 9 Oct 2025 13:54:29 +0200
From: AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>
To: Jason-JH Lin <jason-jh.lin@...iatek.com>, Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>, Jassi Brar <jassisinghbrar@...il.com>,
 Chun-Kuang Hu <chunkuang.hu@...nel.org>,
 Mauro Carvalho Chehab <mchehab@...nel.org>
Cc: Matthias Brugger <matthias.bgg@...il.com>,
 Nicolas Dufresne <nicolas@...fresne.ca>, Nancy Lin <nancy.lin@...iatek.com>,
 Singo Chang <singo.chang@...iatek.com>,
 Paul-PL Chen <paul-pl.chen@...iatek.com>, Moudy Ho <moudy.ho@...iatek.com>,
 Xiandong Wang <xiandong.wang@...iatek.com>,
 Sirius Wang <sirius.wang@...iatek.com>, Fei Shao <fshao@...omium.org>,
 Chen-yu Tsai <wenst@...omium.org>,
 Project_Global_Chrome_Upstream_Group@...iatek.com,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
 dri-devel@...ts.freedesktop.org, linux-mediatek@...ts.infradead.org,
 linux-arm-kernel@...ts.infradead.org, linux-media@...r.kernel.org
Subject: Re: [PATCH v7 13/20] drm/mediatek: Add programming flow for
 unsupported subsys ID hardware

Il 27/08/25 13:37, Jason-JH Lin ha scritto:
> To support hardware without subsys IDs on new SoCs, add a programming
> flow that checks whether the subsys ID is valid.
> 
> If the subsys ID is valid, the flow will call cmdq_pkt_write_subsys()
> instead of the original cmdq_pkt_write().
> 
> If the subsys ID is invalid, the flow will call cmdq_pkt_write_mask_pa()
> to achieve the same functionality.
> 
> Signed-off-by: Jason-JH Lin <jason-jh.lin@...iatek.com>

In mediatek-drm and in mtk-mdp3 the performance of mtk_ddp_write is important: in
both, there are ways to know whether a platform is expected to always use the
cmdq_pkt_write_mask_pa() or the subsys() one.

Please check what platform is this driver running on - based on the platform, you
can assign a function pointer, so that you always call it like

priv->write_cmdq_pkt(cmdq_pkt, cmdq_reg, ofst, val, mask);

write_cmdq_pkt() could point to, either:
1. A function that checks if subsys != CMDQ_SUBSYS_INVALID, for platforms that
    are expected to have mixed PA *and* SUBSYS (hopefully none!!!); or
2. The cmdq_pkt_write_mask_subsys() function; or
3. The cmdq_pkt_write_mask_pa() function.

This removes lots and lots of branches at every call.

I want to remind you that CMDQ packets are being generated in many cases in the
mediatek-drm driver, one of which is upon VBLANK; Think of the case in which we
are driving a high refresh rate display (>=120Hz): not just in DSI Video mode
for which we manage just only vblanks and data pumping (which still needs quite
a bit of GCE writes).. but something like DSI CMD mode would probably generate
*a myriad* of GCE calls.... and that is only one of the cases, there are more
that don't involve specifically DSI.

Of course, for mtk-mdp3 there's a different story - but I guess it's useless to
add a specific example for that, I'm sure you got the point here.

Cheers,
Angelo

> ---
>   drivers/gpu/drm/mediatek/mtk_ddp_comp.c | 24 ++++++++++++++++++------
>   1 file changed, 18 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/gpu/drm/mediatek/mtk_ddp_comp.c b/drivers/gpu/drm/mediatek/mtk_ddp_comp.c
> index ac6620e10262..d902a65e1232 100644
> --- a/drivers/gpu/drm/mediatek/mtk_ddp_comp.c
> +++ b/drivers/gpu/drm/mediatek/mtk_ddp_comp.c
> @@ -66,14 +66,28 @@ struct mtk_ddp_comp_dev {
>   	struct cmdq_client_reg cmdq_reg;
>   };
>   
> +#if IS_REACHABLE(CONFIG_MTK_CMDQ)
> +static int mtk_ddp_write_cmdq_pkt(struct cmdq_pkt *cmdq_pkt, struct cmdq_client_reg *cmdq_reg,
> +				  unsigned int offset, unsigned int value, unsigned int mask)
> +{
> +	offset += cmdq_reg->offset;
> +
> +	if (cmdq_reg->subsys != CMDQ_SUBSYS_INVALID)
> +		return cmdq_pkt_write_mask_subsys(cmdq_pkt, cmdq_reg->subsys, cmdq_reg->pa_base,
> +						  offset, value, mask);
> +	else /* only MMIO access, no need to check mminfro_offset */
> +		return cmdq_pkt_write_mask_pa(cmdq_pkt, cmdq_reg->subsys, cmdq_reg->pa_base,
> +					      offset, value, mask);
> +}
> +#endif
> +
>   void mtk_ddp_write(struct cmdq_pkt *cmdq_pkt, unsigned int value,
>   		   struct cmdq_client_reg *cmdq_reg, void __iomem *regs,
>   		   unsigned int offset)
>   {
>   #if IS_REACHABLE(CONFIG_MTK_CMDQ)
>   	if (cmdq_pkt)
> -		cmdq_pkt_write(cmdq_pkt, cmdq_reg->subsys,
> -			       cmdq_reg->offset + offset, value);
> +		mtk_ddp_write_cmdq_pkt(cmdq_pkt, cmdq_reg, offset, value, GENMASK(31, 0));
>   	else
>   #endif
>   		writel(value, regs + offset);
> @@ -85,8 +99,7 @@ void mtk_ddp_write_relaxed(struct cmdq_pkt *cmdq_pkt, unsigned int value,
>   {
>   #if IS_REACHABLE(CONFIG_MTK_CMDQ)
>   	if (cmdq_pkt)
> -		cmdq_pkt_write(cmdq_pkt, cmdq_reg->subsys,
> -			       cmdq_reg->offset + offset, value);
> +		mtk_ddp_write_cmdq_pkt(cmdq_pkt, cmdq_reg, offset, value, GENMASK(31, 0));
>   	else
>   #endif
>   		writel_relaxed(value, regs + offset);
> @@ -98,8 +111,7 @@ void mtk_ddp_write_mask(struct cmdq_pkt *cmdq_pkt, unsigned int value,
>   {
>   #if IS_REACHABLE(CONFIG_MTK_CMDQ)
>   	if (cmdq_pkt) {
> -		cmdq_pkt_write_mask(cmdq_pkt, cmdq_reg->subsys,
> -				    cmdq_reg->offset + offset, value, mask);
> +		mtk_ddp_write_cmdq_pkt(cmdq_pkt, cmdq_reg, offset, value, mask);
>   	} else {
>   #endif
>   		u32 tmp = readl(regs + offset);