linux-kernel - [PATCH] media: platform: mtk-mdp: Fix mdp_ipi

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Date:   Mon,  7 Mar 2022 16:56:53 +0100
From:   AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>
To:     minghsiu.tsai@...iatek.com
Cc:     houlong.wei@...iatek.com, andrew-ct.chen@...iatek.com,
        mchehab@...nel.org, matthias.bgg@...il.com, hans.verkuil@...co.com,
        linux-media@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kernel@...labora.com, acourbot@...omium.org,
        irui.wang@...iatek.com,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>
Subject: [PATCH] media: platform: mtk-mdp: Fix mdp_ipi_comm structure alignment

The mdp_ipi_comm structure defines a command that is either
PROCESS (start processing) or DEINIT (destroy instance); we
are using this one to send PROCESS or DEINIT commands from Linux
to an MDP instance through a VPU write but, while the first wants
us to stay 4-bytes aligned, the VPU instead requires an 8-bytes
data alignment.

Keeping in mind that these commands are executed immediately
after sending them (hence not chained with others before the
VPU/MDP "actually" start executing), it is fine to simply add
a padding of 4 bytes to this structure: this keeps the same
performance as before, as we're still stack-allocating it,
while avoiding hackery inside of mtk-vpu to ensure alignment
bringing a definitely bigger performance impact.

Fixes: c8eb2d7e8202 ("[media] media: Add Mediatek MDP Driver")
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>
---

This patch has been tested on Acer Chromebook R 13 (MT8173 Elm) on Debian Sid.

This is an alternative solution to the mtk-vpu approach, found here:
https://lore.kernel.org/all/20210920170408.1561-1-dafna.hirschfeld@collabora.com

 drivers/media/platform/mtk-mdp/mtk_mdp_ipi.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/media/platform/mtk-mdp/mtk_mdp_ipi.h b/drivers/media/platform/mtk-mdp/mtk_mdp_ipi.h
index 2cb8cecb3077..b810c96695c8 100644
--- a/drivers/media/platform/mtk-mdp/mtk_mdp_ipi.h
+++ b/drivers/media/platform/mtk-mdp/mtk_mdp_ipi.h
@@ -40,12 +40,14 @@ struct mdp_ipi_init {
  * @ipi_id        : IPI_MDP
  * @ap_inst       : AP mtk_mdp_vpu address
  * @vpu_inst_addr : VPU MDP instance address
+ * @padding       : Alignment padding
  */
 struct mdp_ipi_comm {
 	uint32_t msg_id;
 	uint32_t ipi_id;
 	uint64_t ap_inst;
 	uint32_t vpu_inst_addr;
+	uint32_t padding;
 };

 /**
-- 
2.35.1